Create a VPN Based VOIP Communication for Mobile Users

We have 2 sophos firewall installed XG115, now in the network we have IPPBX installed behind the firewall at 2 locations as enclosed is the network map below. Now as it can be seen that both firewalls are connected through an IP-SEC Tunnel and both the firewalls have an IPPBX behind them. Now the issue is that when we connect from a mobile user through VPN, the mobile user is able to connect to the IP-PBX and register but there seems to be an issue with voice, the issue is that the mobile user is able to hear voice of user1 or user2 but the user1 or user2 is unable to hear mobile users. Also note that user1 and user2 are able to communicate to each other easily without any issues. The requirement for this small project is as below: 1. Both user1 and user2 should be able to communicate to each (as is the case currently) 2. Mobile Users would install an app in their mobile phones and connect to the IP-PBX on [login to view URL] 3. Mobile users should be able to speak to user1 or user2 without issues (as explained this is the problem). 4. Mobile user if connected to the network from within should be able to still communicate. 5. Ideally the VPN should be seamless meaning should be always connected to the server and there should not be any need for making changes if the mobile user comes inside the main network.