SIP
180 Ringing
The UA receiving the INVITE is trying to alert the user. This response is used to initiate local ringback tone at the caller's end.
Когда вы видите этот код
During call setup when the callee's phone is ringing. You should hear a ringback tone after receiving this response.
Как исправить
No fix needed — the callee's device is ringing. If the call never connects after ringing, check the callee's device or call forwarding settings.