I have a question
I am going to install VOIP GSM gateway,
Between call manager and this gateway will be SIP trunk,
it maybe stupid question,
but SCCP phones can call via this SIP trunk?
As long as you are running CallManager 4 or later then this should be ok.
I would recommend that wahtever GSM gateway you buy be certified as compatible with CallManager so that if you run into difficulties you can get assistance from Cisco TAC and the GSM manufacturer.
The QuesCom 400 IP/GSM would be an example of a certified product.