As you are using GPRS, you should be aware that, generally, the IP-address of your devices will not be fixed. That means it is extremely difficult, if not impossible, to have the device as a slave.
I, at least, found it much easier to have it the other way round and have the devices connect to a server when they have data to send.

It may be possible to have your provider issue fixed IP-addresses for your devices, but unless they can really guarantee the addresses will never change, I would choose the alternative route.


Regards,
Rob.