Sip Trunk In

We have Sip trunk in from Gradwell in the UK.
the trunk is just forwarded to the PBX and we define it with an inbound route and direct it to where we want it to go.
Problem is we want to use our G729 codec with the sip trunk in, we use IAX2 for outbound so that is easy, how do we define this for an inbound trunk that is just a forwarded sip trunk ???
Thanks for any help…