With MGCP, Callmanager is in control so the translation profile, etc does not get used. You'll need to use the options available on the call manager side or change the gateway over to a H.323 configuration.
In Call Manager you can set it to prefix digits on to the calling party number by going under call manager service parameters, then advanced, and there will 3 or 4 options to prefix numbers on to calls received for mgcp gateways. The options I believe (offhand) are international prefix, national prefix, subscriber prefix, and unknown prefix. This is 4 ISDN numbering types basically. These options are available since 3.3(4) I believe.