Weird IVR audio problem

I have installed FreePBX, no problems. I have inbound and outbound SIP calls working with audio both ways. All good. All the right ports open.

If I redirect my inbound route to the IVR solution, it does not play the audio or recognised the DTMF tones. My SIP provider tells me their servers use the rfc2833 codec so that is what I have configured in the trunk.

What is weird is that if I make a 7777 call, emulating an outside call I can hear the IVR announcement just fine, and the DTMF tones work and route correctly.

SIP provider is relucant to help.

Any ideas what I can do to diagnose? Wireshark shows the SIP and RTP connections being established and the RTP packets flowing out of the gateway. Really really stumped.