Hi Vick,
We had the same issue a couple of weeks ago on our SIP trunks. We are using Interoute (SIP provider) for placing our calls. About 80% of those calls had 'no ringback'. After contacting Interoute, we found that the changing 'Media Method' (trunk setting) to 'Delayed' instead of 'Normal' (which means early media) did solve the issue. However, this issue did not exist on our WebRTC connections, only the SIP trunks.

Interoute responded with: "
Lync does not support early media. Sometimes upon receiving an incoming call the server sends back 183 Session Progress message with SDP details. Normally when 183 with SDP is sent to the caller it is assumed that an early media channel is opened and the RBT is expected to be played by the B-end via it. However, as mentioned above early media is not supported by Lync/SfB and this scneario results in a blank media channel opened with no RBT being played from the server, just silence."
As mentioned, this setting does not appear on our WebRTC trunk so not sure if this helps.
I also understood the 'ringback issue' might be Telco related.
Good luck!
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Christian Ropertz
Dealerdirect B.V.
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