Connecting a legacy PBX to a SIP trunk
Published
Connecting a legacy PBX — an Alcatel OXO or OXE, for example — to a SIP trunk lets a company take the tariffs and the flexibility of VoIP while keeping the phone system it has and the habits its users have. The same gateway later becomes the stepping stone for a gradual migration to an IP PBX.
The principle is the same whichever provider is involved. beroNet OS ships provider templates for the trunks that have been tested; they are listed under Provider partners.
Before ordering, one thing has to be known: how the PBX is connected — ISDN (BRI or PRI) or analogue. That decides whether the site needs an ISDN gateway or an analogue gateway. The configuration then takes three steps.
1. The physical ports
In the hardware section the ports are set to NT mode: the gateway takes the role the provider used to have, and the PBX carries on seeing itself as TE exactly as before. The ports are then collected into a port group on the PSTN side, which is what the dialplan will address.
On beroNet gateways TE and NT are a software setting, pin swap and 100 ohm termination included. That is the difference between a browser tab and a second site visit when a building turns out to be wired the other way round from the drawing.
2. The SIP trunk
In the SIP section the provider trunk is declared. beroNet OS carries provider templates, so registration parameters, domain and the known quirks are pre-filled rather than guessed.
One point that usually surprises people in a good way: calls between the ISDN or analogue ports and SIP consume no session licence. Licences cover SIP-to-SIP traffic only.
3. The dialplan
The dialplan joins the two worlds, and it is the fiddly part. It is also where number lengths are adjusted: an OXO often expects only four or six digits, while the gateway receives nine or ten from the provider. Digits are truncated towards the PBX and completed towards the provider, in both directions.
The syntax and the matching logic are documented in the beroNet Wiki.
With the PSTN switch-off running through one market after another, this remains the most common installation there is: the line becomes IP, the PBX stays, and the gateway absorbs the difference. The other common starting points are collected under Solutions.