So, I just did this deployment where the SIP Trunk was connected to an AudioCodes Mediant 800 gateway. Everything worked just fine until some callers told us that when outside callers started calling the phone numbers that were assigned or transferred to analog devices, there was no ringback until the call is answered. When I checked it out, if found that even if there is no ringback, the analog phone is ringing.
The analog phones were connected to the FXS ports of an AudioCodes MP-114 gateways. Basically, this is how it works:
The call comes through Mediant 800 to a Response Group, and the members of the Response Group transfer the call to an analog device.
During the troubleshoot, I checked on the SIP Definitions for the status of the Play Ringback configuration. It seemed like everything was okay so far.
After that, I changed the 183 Message Behavior from Progress to Alert.
It didn’t help. There was still no ringback.
After some more troubleshooting, I remembered that the Early Media could create problem if enabled. I checked it out to see if it was enabled, and it was, so I disabled it.
After that, all of the outside calls had ringback, and my problem was solved.