Control.com - Nerds in Control

GPRS usage for two way SCADA communications applications - slave to master is understandable in a way that the slave with GPRS connectivity (and a dynamic IP address of course!) will log on to a static server's IP address (that of the master with a static IP address provided by the Internet Service Provider) and pushes the data to the master. However, the master to slave (RTU) communication is not possible without a static IP address within the slave. This is being achieved through SMS (control signal command) from master to slave on its MIN (Mobile Identification Number).

I would like to understand how this is done, and if there is a possibility of Master/Slave communication without the SMS route and without a static IP address at the slave.

Thanks in advance.