The usage of GSM phone as GSM modem to dispatch data SMSs is applicable if you have small number of trackers.
since you have just one GSM channel, you may dispatch just one SMS message at one time, assume that dispatching of text message (SMS) will takes 2 sec and we wish to get every 1 minute update of trackers position.
So the system equipped by only one GSM modem can handle just 30 trackers simultaneously.
The calculation is very simple:
Total Number of Devices = ( Position Acquisition Frequency ) / ( Text Message Dispatching Time )
So to build Online Tracking Application based on SMS messages with large number of trackers it is no good idea to use GSM modem. Instead try some M2M service from your Mobile Operator.