Domain in Contact header?

Hi,

I’m running Asterisk 1.2 (trixbox + a load of customisations).
We are connecting to a new SIP peer and they require that the “Contact” SIP header contains the domain rather than the IP address.

For example, at the moment it is:
Contact: sip:+44xxxxxxxxxx@64.69.yyy.zzz

Whereas they want it to be:
Contact: sip:+44xxxxxxxxxx@sip.mydomain.com

Is there any way of forcing this? I have fromdomain set in the peer definition already.

Thanks

JKI