How to Choose an IP-PBX: 8 Questions to Ask Before You Buy

Choosing an IP-PBX comes down to four numbers you can verify on a product page, four questions about the analogue side of your estate, and one honest conversation about what happens when the system is full. Model names and feature checklists come afterwards. Start with the requirements, because a spec sheet will tell you how many concurrent calls a platform can handle without telling you whether it can terminate the analog handsets already sitting in your building.

Definition: An IP-PBX is a converged telephony system that registers SIP endpoints, handles internal call routing, and connects to outside lines through SIP trunks, FXO ports or dedicated media gateways. A PBX selection is the process of matching a platform's registered users, IP extension count, trunk capacity, built-in FXS/FXO port count and concurrent call rating to the site it has to serve.

Start With Requirements, Not Model Numbers

Most buying processes start the wrong way round. A shortlist gets assembled from product names, and then the requirements are fitted to whatever survived. That order reliably produces a system that is either oversized on one axis and short on another.

The requirement sheet that goes to every vendor

Write this down before you look at a single product page, and send the same sheet to every vendor. It should contain, at minimum:

• Extension count by type. IP desk phones, IP softphones, analog handsets, DECT handsets, fax points and any other endpoints that need a line.
• Trunk count and type. How many simultaneous outside lines, on what bearer. SIP trunk account count, ISDN PRI channel count or analogue PSTN lines, and how many of each are needed for redundancy rather than for capacity.
• Analog endpoint count. This is the one most sites underestimate, because analog devices include intercoms, fax machines, back-of-house handsets and legacy equipment that nobody has audited since installation.
• Concurrent call expectation. Not user count. A site can have many registered users and a much smaller number of simultaneous calls.
• Recording, queuing and IVR requirements. Whether recording is mandatory, where it is stored, how long it is kept, and who needs to retrieve it.
• Survivability requirement. What must keep working when the WAN, the server or the power fails.

Send that sheet to three or four vendors and you will find that the differences in the answers are more informative than the differences in the marketing. One vendor will answer with a single headline number when asked about concurrent calls; another will ask what "concurrent" means to you before answering.

Users vs Extensions vs Trunks: Reading the Spec Sheet

These three numbers are published on New Rock's OM series pages, and they are not interchangeable. Reading them correctly is most of what selection requires.

Why the three numbers differ

• Max Users covers every entity that can register or be counted as a user, including IP extensions, analog extensions registered through FXS ports, and trunks.
• IP Extensions is the subset of that population that is an IP endpoint.
• Trunks is the count of outside-line connections, whether SIP trunk accounts, ISDN channels or analog FXO lines.

The gap between Max Users and IP Extensions is a direct read on how much analog equipment the platform expects to carry. On the OM20G, 54 max users against 50 IP extensions leaves room for the small number of entities that are not IP endpoints, and the same model carries 4 built-in FXS/FXO ports for the analogue endpoints themselves. The gap widens as you move up the range in a way that tells you how each platform was intended to be deployed.

Why Built-in FXS/FXO Changes the Total Cost

This is the question that most often gets skipped during selection, and it is the one that changes the bill of materials.

The difference between a port and a box

An FXS port is an analog line interface built into the IP-PBX. An FXS/FXO gateway is a separate device that does the same line termination and then connects to the IP-PBX over SIP. Functionally similar. Commercially and operationally different: the built-in port needs no separate box, no separate power supply, no separate uplink and no separate configuration to keep in step with the PBX.

New Rock publishes built-in analogue port counts as a headline specification for the OM series, ranging from 4 on the OM20G through 12 on the OM50G, 48 on the OM80E and 96 on the OM200G and OM200G-A. Where a property's analog endpoint count fits inside the built-in capacity of a single platform, the gateway line items disappear from the quote entirely.

Where it does not fit, gateways are the right answer, and the published port tiers define the granularity available:

The OM20G all-in-one IP telephony system is the smallest platform that carries built-in FXS/FXO, with up to 4 such ports alongside PSTN, analog phone, SIP trunk, SIP phone and IMS support. The OM200G covers larger sites with 96 built-in FXS/FXO ports. The MX8G gateway adds 8 FXS/FXO ports with one WAN port and one PC port, both 10/100/1000Mbps, and the MX60E-G gateway scales to up to 48 FXS/FXO ports with SIP registration failover, load balancing, PSTN failover and optional 1+1 redundancy on network port and power supply.

Concurrent Calls and Real Headroom

Concurrent call rating is the number that determines whether a platform will carry the site during its busy period, and it is published for every model. It is also the number buyers are most likely to misjudge.

What the rating actually tells you. Concurrent calls is a ceiling on simultaneous conversations the platform can handle, and it is not the same as user count. A small office can have far more registered users than concurrent calls, and a call centre with forty agents can be served by a platform whose concurrent rating is well below its user count because agents are not all on calls at once.

Two practical points follow. First, size for concurrent calls against the traffic you actually see, then leave margin above that figure rather than sizing to the maximum possible. Second, check the ratio between the concurrent rating and the max user figure: a platform rated for 24 concurrent calls against 54 users is a different product from one rated for 90 concurrent calls against 216 users, and the second is the one for sites where the phone is the main channel of work.

The SBC layer and the media boundary. Where multiple sites or an IMS network are involved, the media boundary is a separate decision with separate capacity numbers. The SX100 SBC supports 1000 user registrations, 120 concurrent sessions and 70 media transcoding channels, along with SIP/IMS access, branch networking, inter-network interconnection, signalling header field customisation, codec and protocol conversion, DTMF mode conversion, call recording, voice quality monitoring, trunk redundancy and load balancing. Its codec list runs from G.711 through G.722.2 to AMR and H.264/H.265, and it handles IPv4/IPv6, UDP/TCP, RTP/RTCP/MSRP, TLS/SRTP and SIP/IMS.

Media transcoding is the part that surprises people. If the external side expects a different codec set from the internal side, transcoding capacity becomes a real constraint, and a session count that looks generous can still be limited by the transcoding path. Ask for both numbers.

A Selection Table Built From Published Specifications

Here is the OM series comparison exactly as published by the manufacturer. Every figure below is a vendor specification, which is what makes it usable as a starting point for a shortlist rather than a marketing comparison of different brands.

Two patterns are visible in the table. The first eight-model range is not a single family but two: the G models with built-in FXS/FXO, and the -UC, -A-UC and OM8000 models with no built-in analog ports at all, which means every analog endpoint on those platforms needs an external gateway. The second pattern is in the concurrent column, which rises with model size but not proportionally to max users.

The manufacturer also publishes the intended size brackets alongside the model names: OM20G (under 20), OM50G (under 50), OM80E-G (under 200), OM200G (under 500), OM200G-A, OM500 (under 1000), OM1000-UC (under 1000) and OM8000 (under 10000). Treat those brackets as guidance about the tier a vendor designed the box for, and treat the table as the hard specification.

Eight Questions to Ask Before You Buy

These are the questions that separate a platform that fits from a platform that has to be worked around. Ask all eight, and ask them of every vendor on the list.

1. How many concurrent calls does the platform actually support, and how is that figure defined? If the answer is a different number from the user count without explanation, ask which traffic the rating assumes.
2. How many FXS ports are built in, and how many more can I add without a separate box? Get the built-in count as a published number and the gateway option listed alongside it.
3. What happens to my existing analog handsets? Confirm whether hook switch status, message waiting, caller ID and DND pass through on the analog ports, and whether the handsets need firmware or replacement.
4. How do I connect to the outside world, and what happens if that link fails? Ask about SIP trunk failover, PSTN failover and whether failover is on the platform, on the gateway, or on both.
5. What survives a WAN or server outage, and what does not? Ask for the specific list of services that keep working locally. Any survivability claim should come with a local component in the design.
6. Where is recording stored, and how do I retrieve it? Ask about local storage on the platform, central recording management, and the export format.
7. How is a multi-site or IMS deployment handled? Establish whether an SBC sits in the design, what its session and transcoding limits are, and where the media boundary falls.
8. How do I administer this across more than one site? Centralised management and remote operations support matter once there is more than one room to visit.

IP-PBX Buying Guide FAQ

What is the difference between max users and IP extensions?

Max users is the total count the platform accommodates across all entity types, including IP extensions, analog extensions on FXS ports and trunks. IP extensions is the number of those entities that are IP endpoints. Where the two figures differ, the difference is roughly the size of the analog and trunk population the platform is designed for.

Does a larger model automatically mean a better system?

No, and picking by size is the commonest selection error. A larger model brings more concurrent call capacity and more ports, but it also brings more to configure, license and maintain. The OM80E at 216 max users, 48 built-in FXS/FXO ports and 90 concurrent calls serves many mid-size estates better than the OM200G would.

Do I need built-in FXS/FXO at all?

Only if you have analog endpoints to terminate. If the estate is entirely SIP endpoints, the models without built-in analog ports leave you with fewer boxes to install and no gateways to power and configure. If there are analog handsets, intercoms or fax points, the built-in ports remove the need for separate gateway hardware.

How much concurrency headroom should I plan for?

Plan against the traffic you actually observe, then leave margin above it. The published concurrent call ratings differ sharply in their relationship to user count across the range, so pick the model whose rating fits your busy period rather than the model with the largest user number.

Should I use a gateway or a platform with built-in ports?

Use built-in ports where the analog endpoint count fits inside the platform's published FXS/FXO capacity. Move to gateways when it does not, or when analog endpoints sit on floors the platform does not reach. The MX60E-G at up to 48 FXS/FXO ports is the mid-point tier, and the MX120G at up to 96 covers multi-tenant and large call centre estates.

Do I need an SBC in the design?

It depends on whether you have more than one location, an IMS network, or a need for codec and protocol conversion at the boundary. Where you do, treat the SBC's session count and media transcoding capacity as separate limits to be sized, not as one number. The SX100 provides 1000 user registrations, 120 concurrent sessions and 70 media transcoding channels.

Can one platform manage a large group of sites?

The OM series scales from 54 max users on the OM20G to 10000 on the OM8000, and the OM8000 is specified with 800 concurrent calls. UMS unified management software provides unified management and remote operations for OM series IP-PBX systems and MX series VoIP gateways together, which is what makes multi-site administration practical.

Conclusion

A good IP-PBX selection produces a platform whose four published numbers — users, IP extensions, trunks and concurrent calls — line up with your requirement sheet, and whose built-in FXS/FXO count either covers your analog estate or leaves you choosing the right gateway tier deliberately. Every figure you need is on the product page; nothing about the selection should depend on a vendor estimate.

The concrete next step is to complete the requirement sheet above, put it in front of three or four vendors, and compare the answers to the published specification tables rather than to each other. When the shortlist is down to two platforms, the differentiator is usually the analog side and the failover behaviour, so test those two areas hardest before signing anything.