I have a US flynumber and the Acrobits app.
Receiving calls is fine.
Calling US numbers from Acrobits is fine. (Outgoing calls are enabled in PBX.)
But if I try to call a UK number, it connects but I can’t hear anything and neither can the other person.
Can you try a different network location just in case, I would also try restarting the app
If that doesn’t do the trick please also try different codecs on the app. Give priority to G711 ULAW and/or ALAW. Try GSM as well.
Finally, make sure STUN and sRTP is off in the SIP app.
Worst case I would try a new set of SIP credentials from the PBX and enter into the app.
Please let us know if this persists as we can debug from here
Note: Another tip is to toggle the “transport” ( also a setting in all SIP clients ) from UDP to TCP or vice versa