An interesting topic. As you know the GPRS / Data network is designed for handsets and as such is today a network device initiated connection. This is all well and good for handsets but for machine to machine
So how do we get our remote device to initiate a data session on request. There are two ways to do this, both similar but utilising different delivery mechanisms. We use a voice call or sms - commonly known as a 'shoulder tap' or 'wake up' call.
Voice wake up - set up the remote device to respond to a voice call (generally one that is not completed) by establishing a data session so that the server can send data to the remote device.
SMS wake up - this is very similar to the voice wake up, but it is possible to pass an instruction in the body of the SMS that can trigger the device to do something specific.
Obviously cost considerations will drive which is most suitable for you.
Hope this helps.