I have a new FreePBX install
FreePBX installed from distro downloaded last week
Flowroute SIP Trunk with DID
3Com OfficeConnect VPN Firewall
Public static IP address
Firewall has had various ports forwarded in troubleshooting ranging from just SIP ports (10000-20000, 5060) and in testing, everything wide open.
Outbound calls = fine
Inbound calls to phone = fine
Inbound calls to phone transferred to voicemail = fine
Inbound calls to voicemail directly (DND or Ring No Answer) - voicemail audio is not there. A 0 second message is recorded, no audio on either side.
Issue appears to be - SIP is not “connecting” to the freePBX voicemail, but SIP is connecting to phones, and when phones transfer the call back to freePBX to handle voicemail - connection occurs.
In various testing (changing IP addresses, re-registering SIP connection with FlowRoute, etc) - I got it to work for a few minutes.
I think the 3Com firewall might be to blame, although all ports are open at this point.
System worked on a residential DSL just fine in initial testing, with dynamic IP addresses with the freePBX configured for the public ip address of the DSL modem.
I found this issue unanswered in a few other forum posts. This post closely resembles my problem (had to add spaces since links are not allowed by new users):
community dot freepbx dot org/t/voicemail-audio-issue/13827