Perhaps forwarding your RTP ports would solve the problem.
1) Use your router to assign a static IP to each ATA.
2) Navigate to Service Providers >> ITSP Profile
X >> RTP. Set each ATA to use a different LocalPortMin and LocalPortMax range, for example, 20000-20010 for the first OBi and 20011-20020 for the second.
3) Use your router to forward the
UDP ports 20000-20010 to the first OBi's IP address, and 20011-20020 to the second OBi's IP address.
4) Test to see if the problem is solved.
Alternate solutions: use the OBi's dial plan to route calls to the other ATA's phone number via OBiTALK. Or, try to use
voip3.freephoneline.ca - if that works, file a ticket with FPL and advise that you have no audio unless you use voip3.