I think if you turn off the EDGE function, you might facing call drops problem like my friend did.

He was a 'M' prepaid user and he off the EDGE function (to save cost) after jailbreak his 3GS , since then his calls always suddenly drop or disconnected. And he was being advised by the service provider to change to post paid package with free data usage.

What me and my friend understanding is, this local service provider enable the EDGE function just to strengthen the signal, but I don't why, as neighboring country's service provider no need to do so, and, the most important, their iPhone battery usage are longer than ours iPhone here.