There are typically four ways to do this:
- IP based trunking if the PBX supports it. This would involve using protocols such as SIP, H.323, MGCP, etc
- T1 tie trunks. You can set up a QSIG or PRI connection between the two phone systems. This configuration requires that you have hardware on both ends to support the T1 connection.
- Analog lines. You can extend individual analog lines between your phone systems. This requires that you have hardware to support it as well as making sure you have enough lines to support the number of calls you will route between phone systems at any given time.
- Forwarding calls out of your telephone system’s PSTN conneciton. In this scenario, you could allow users on the old PBX to dial the typical 3-5 digit extension and simply send the call out to your telco so that it comes in via the PSTN on the other phone system. This is probably the easiest solution but you could incur carrier charges depending on your set up.