I'm using iPhone 3G now and according to iPhone's manual, switching off 3G (Settings > General > Network > Enable 3G > Off) will save significant battery power. iPhone will then use the slower GPRS (or EGDE?) for data access (online).

I read somewhere we won't be able to receive voice calls or SMS (and MMS too?) if we use GPRS to go online. Is this true? What about EDGE?

So, if I want to go online and at the same time be able to make or receive calls/SMS/MMS, I must use 3G?