[VoiceOps] Mobile call forwarding media issues

[Calvin E.]

I receive a majority of traffic via remote call forwarding from mobile
carriers. A small number of subscribers, or specific callers to those
subscribers, experience broken calls. The symptoms vary, but they are media
related:
Callers hear rings then silence.
Callers hear rings then beeps, or busy tones.
Callers hear continuous ringing.
Callers hear rings then call drops; not sure if this report is the same as
first, not everyone checks if the call duration is still incrementing.

We are receiving these calls via SIP, and in all of the above cases we
received the call and signaled a brief ringing (180 w/o SDP) followed by a
recorded greeting (200 OK w/SDP). Sometimes we hear the caller, sometimes
only line noise, sometimes no RTP at all or silent RTP (all binary 1).

One hypothesis is these calls are failing due to the change in media from
the mobile carrier's ringing to our greeting. It's a terrible experience
for our subscribers to be told this is a problem with their mobile
carrier's call forwarding, or the original calling carrier, or some carrier
in between.

I'm curious if others have experienced this, and if they were able to
mitigate it from the terminating side via signaling, (UPDATE or re-INVITE,
repeating our original SDP), or if this has to be fixed upstream.
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