I know this is an old thread, but wanted to post my findings there. I have come across the same issue and have tried various things to try and resolve it. I don’t think it’s an RTP issue, inbound calls work fine, it’s just when calls are forwarded back out that audio/RTP doesn’t get passed along.
Setting “Fax Detection” on inbound routes also resolved the issue for me. Curiously, I looked at the dialplan to figure out what’s going on and why that’s “fixing” the issue. It’s not so much that “Fax Detection” fixes it, but that it adds “Answer” to the dialplan. I confirmed this by turning off “Fax Detection”, then overriding the route in the conf file and adding "Answer in the route.
Example (in extensions_override_freepbx.conf):
[ext-did-0002]
exten => 5551231234,1,Set(__DIRECTION=INBOUND)
exten => 5551231234,n,Gosub(sub-record-check,s,1(in,${EXTEN},dontcare))
exten => 5551231234,n,Gosub(app-blacklist-check,s,1())
exten => 5551231234,n,Set(__FROM_DID=${EXTEN})
exten => 5551231234,n,Set(CDR(did)=${FROM_DID})
exten => 5551231234,n,ExecIf($[ “${CALLERID(name)}” = “” ] ?Set(CALLERID(name)=${CALLERID(num)}))
exten => 5551231234,n,Set(CHANNEL(musicclass)=default)
exten => 5551231234,n,Set(__MOHCLASS=default)
exten => 5551231234,n,Answer
exten => 5551231234,n,GotoIf($[“${__REVERSAL_REJECT}”=“TRUE” & “${CHANNEL(reversecharge)}”=“1” ]?macro-hangupcall)
exten => 5551231234,n,Set(__CALLINGPRES_SV=${CALLERPRES()})
exten => 5551231234,n,Set(CALLERPRES()=allowed_not_screened)
exten => 5551231234,n(dest-ext),Goto(from-did-direct,1234,1)
I think the underlying issue is still RTP related, but having the system answer the call before sending it on seems to have resolved the issue for me.