IP phone not registering via VPN (ipsec)?

Question

Network administration

Hello,
has Astrtisk (FreePBX) configured and hundreds of phones on the same physical subnet. Everything is set up and working without problems. The client had a need to put a new IP phone in a remote office. VPN (ipsec) is raised between offices by means of PFSENSE at both ends of the office. The phone has a different subnet address. The phone does not register through the VPN. However, there is an entrance to the web-interface of the remote phone from the head office (pings go in both directions). Tell me, can someone come across such a problem? Where to dig?