Here are the rules of engagement when it comes to ring back:
- If Lync receives a SIP 180 Ringing message, it signals to the Lync client to play a ringing tone. These tones are not transmitted from the callee to the caller in RTP media packets; the ringing tone is generated locally by the Lync client
- If a SIP 180 Ringing response is not received, never generate local ringing
- If a SIP 180 Ringing is received but there are no incoming media packets, generate local ringing
- If a SIP 180 Ringing has been received and there are incoming media packets, play them and do not generate local ringing
Great article here if you want to learn more.