Yes you should be able to do this, configure the a dial-peer with session protocol sipv2 for the ITSP and the address. Under sip-ua configure username/password/realm needed for http digest authentication to the ITSP.
If the GSM adapter is capable of registration and authentication, configure "voice register pool xx" again with username and password. A mac addrss is required but can be all zeros.
If not capable of registration, configure a static dial-peer similarly to the one mentioned above.
Can you share details on the type of GSM adapter you are using supporting SIP ?
Hope this helps, please rate post if it does!
