Attempted UDI video call appearing on Orange handset as normal voice call

Hi,

A system we have at work acts as a gateway to H.324m and has no
trouble whatsoever dialing Vodafone and O2 mobiles over our UDI-bearer
ISDN BRI line. However, whenever we call an Orange subscriber (who
has 3G coverage and can accept calls from the Vodafone and O2
subscribers), the call presents as a normal voice call which, when
answered, sounds like squealing and noise.

Trouble is, we're not sure who to point the finger at:

1/ The vendor of the Gateway (but then, why does it work for the other
networks we've tried as destination)
2/ BT as our ISDN provider
3/ Orange as the receiving network
4/ Some third party that's acting as a signaling gateway between BT's
ISDN network and Orange's network

Any thoughts?

(Interestingly, calls in the other direction work fine!)