[Freeswitch-users] Contact header in gateway invite

Hi, I have the following issue: a carrier wants the A number in the contact field i.e.: Contact: <sip:+31xxxxxxxxx@ip.ad.dr.ess;[add. params]> I know; the contact is not up to the called party but they want it anyway. I spent quite some time, and I found some earlier mailing list threads but no answer that solves my particular problem. I noticed that by setting the extension parameter in the gateway configuration, this results in the desired header format. However, this is not dynamic and therefore not the A number (freeswitch runs in a SBC configuration, passing traffic in and out of the network). Is there any way to either pass the extension= parameter from the dialplan or somehow change the contact header? Thanks, Remco