Iphone and the Sim card?

I am thinking about getting an iphone, but i was wondering if I end of getting one, can I take the sim card out and put it in another divice as long as it is at&t divice? Then of corse be able to put it back in with no problems. If yes will everything work if the older phone supports it, (3G and texting and calling)

I was wondering if I can do this, so I can get one of my old phones and take it with me on nonelectronic friendly trips. so if it happens to fall in the sand or water or off a cliff, ect. I am not going to be looking for a new iphone, just a new sim card and old phone.

Thanks