It’s possible to have 2 GSM modems dial one another to set up a data connection. If there is a central office, with, say, 100 satellite modems, having the office modem dial each satellite modem (or vice-versa) in turn is not a good solution.
If the satellites could dial a number for which there could be numerous (virtual) lines then that would be an advantage. A VOIP phone system would appear to fit the bill for this. Are there any standard products that provide such a dial-up service that would allow numerous remote GSM modems to connect to an office LAN using CSD?