Firstly, this whole network setup is inherited and has little documentation, coupled with the fact that I'm not familiar with the phone system in question.
Phone system: FreePBX
Handsets: Aastra 9133i
The network used to be a simple single subnet: 192.168.1.* as it occupied a single office location.
They have since moved some of their operation to an new office. The existing office is still on 192.168.1.* and the new office is on 192.168.2.* There is an IPsec VPN tunnel between the 2 sites with all traffic allowed over this tunnel.
The phones left at the old office (where the phone system resides) work fine.
The phones at the new office are able to connect to the phone system and get config changes etc no problem.
I can make and receive calls from the phones in the new office, however:
Internal call extension to extension: No audio at all
Outbound call from extension to my mobile: The caller can hear me, but I cannot hear them.
Inbound call from my mobile to their extension: No audio at all.
The phone system utilises SIP services that point at the Fortigate firewall at the old office.
I suspect this is a firewall or routing issue somewhere but have no idea where to even start with this so any help appreciated.