Migrating to a VoIP phone system
Published
The PSTN switch-off forces classic phone systems onto VoIP: the analogue and digital technologies they were built on are being withdrawn. For a large company that migration can be awkward. From about a hundred employees upwards a gradual migration is the better plan — it gives people time to get used to the new system and to what it can do.
That kind of migration is done with a VoIP gateway and, where telephony is business critical, a failover switch, so that the configuration can be worked on without putting production at risk. The fiddly part is the gateway configuration, and above all its dialplan.
What to know before starting
How the PBX works. Its dialplan is the single most important piece of information: it has to be reflected in the gateway configuration, number lengths included.
What people actually do. Part of the point of a gradual migration is not to impose completely different habits overnight. What worked has to keep working while the change is under way.
The extension numbers. Keeping the same numbering format in the new IP PBX saves rewriting numbers in both directions for the whole length of the project.
The connections in use. BRI or PRI, or analogue: that is what decides whether the site needs an ISDN gateway or an analogue gateway.
Brands do not matter, codecs do
The make of the PBX and the make of the IP PBX do not come into it — the gateways adapt because they are modular. Codecs deserve more attention. Transcoding uses DSP resources, and how many transcoded sessions a device supports depends on the model: an M housing does not offer as many as an L or an XL. If the two systems share no common codec, that belongs in the sizing decision.
Calls between the ISDN or analogue ports and SIP, on the other hand, consume no session licence at all. Only SIP-to-SIP sessions are licensed.
During the transition
The value of the gateway is that it lets both worlds coexist. The old PBX and the new IP PBX stay reachable from each other, external calls keep arriving in the same place, and departments move when they are ready. A worked example is in migrating from an Avaya phone system to 3CX.
For hardware sizing our pre-sales team answers through the contact form; guides and downloads are under Resources.