Here is the scenarios Quadro 16x with latest firmware.

1. Call comes in via external SIP provider...using a G729 codec..sits behind a simple ADSL modem/firewall with all ports NATed to Quadro box
2. Answered by virtual extension 99 (setup for the external SIP provider)
3. Forwards call to auto attendant - 00
4. Message is played (and heard) to dial the required extension. When you do so the call is forwarded to extension (a SIP phone).
5. Upon answering the call on the SIP phone the call is dropped (with call statistics reporting "Not supported")

Now here is the confusing part.

If I call via a landline/mobile to our DID the above scenario happens (ie drops call) but...

If I call via the SIP provider to the DID (still dialing a real number... not a SIP number)... the call comes in the same way but... connected without any problems.

Delving into the logs here is the point of failure:
.776 AcceptMedia: line #21[0], incoming media list is empty.
.776 <<ERROR>> OnSIPCallAccept: line #21[0](e:27), no acceptable media parameters for call with ID 5965938891037978457.

Just for the record the SIP phone handsets are Yealink T28P with the latest firmware Preferred codecs all round are G729

Any ideas?