The VG248 uses SCCP (Skinny) signaling to talk to CallManager for each of the 48 ports... not MGCP.
As far as Extension Mobility is concerned, currently the login and logout mechanisms are based on XML services for the 7960/7940 phones.
If you configure a Cisco VGC phone in CallManager 3.1 you will notice that there is no way to enable the Extension Mobility feature.
If you have a business case that requires this functionality, please contact your account team.
