no, in CCM with MGCP, all dial plan resides on CCM, MGCP gateway is not inteligent. it behaves as a client-server model, so you configure all dial plan stuff on CCM not mgcp.
you access CCM gui interface by:
http:///ccmadmin
you can add the required prefix under route pattern:
go to call routing-> route/hunt-> route pattern-> add your route pattern with the required settings, in the field (prefix digits (outgoing calls) add the required prefix. and thats all.
hth
/majdi