sim card & freedompop

questions:

1) Sim card for iPhone 4. I gather that the sim cards for the iPhone 4 on the sprint network work with only the specified model of iPhone. Is this true? Is the sim card also tied to what version of IOS the phone has?

2) I am attempting to get an iPhone for the FreedomPop.com network. I am wondering how the iPhone 4 will work on the freedompop network.

3) On the FreedomPop network, do I have to use a special app to talk? Do I use the phone in the normal manor for voice? That is will voice use the voice network? [ see 4 ]

4) There was some post in the freedompop forum about voice is voip. Hence, voice would be slow over a 3g network. Any comments?

[ I cannot seem to get an account on freedom pop to ask my questions, so I'm trying here. ]

Robert