In some scenarios of outbound call, there is no ringback tone when calling a specific operator and this might be thought as a failed call. In Yeastar S-Series PBX, there is "Inband Progress" option which would let PBX server generate an inband tone in asterisk and send to phones so they will hear a virtual ringback tone.
Definition from Asterisk Wiki
- If set to yes, chan_pjsip will send a 183 Session Progress when told to indicate ringing and will immediately start sending ringing as audio.
- If set to no, chan_pjsip will send a 180 Ringing when told to indicate ringing and will NOT send it as audio.
In latest firmware version with 188.8.131.52, there are two "Inband Progress" options in PBX, one is in the menu General->SIP->Advanced and the other on is in the menu Trunks->Edit->Advanced.
For outbound call with no ringback tone on the phone, it is typically due to the PBX received 183 Session Progress then received 180 Ringing a little time later with same SDP details from provider. Actually the provider doesn't send the progress tone. To play early media on the phone, it should be 183 progress, if it receives a 180 Ringing later. The issue would happen.
In this case if you check the option Inband Progress under page General->SIP->Advanced->Inband Progress, the PBX would convert 180 Ringing to 183 Session Progress. So that the IP phone will receive 2 183 Progress which it is acceptable for the IP phone and it will play ringback tone generated by PBX.
For the other option in the menu Trunks->Edit->Advanced. Firstly, this option only take effect on inbound calls. If incoming call caller reported there is no ringback tone on their end, try to enable it in trunk page, then asterisk will send 183 back to provider to tell them send ringback tone to the callers. It could be useful when interconnecting with other SIP PBX or gateway.
While usually the provider would handle the ring back tone without this option.