Can you switch SIM cards between iPhone 3GS and another iPhone 3GS?

Me and my boyfriend are switching iPhones because mines cracked and he said hell use it till I get a new screen. We have the same kind of iPhone, both 3GS. I was wondering, if we could just simply switch SIM cards. Like me move mine to his phone, and him move his to my phone. And the phones actually work. If we do so, will our numbers automatically switch over? And what about contacts and what not. I'm mainly worried about our numbers.

Funny thing about iPhones, they tend to stay locked to their specific SIM card. Try it out, if it doesn't work then take it to his carriers store and see if they'll unlock or switch the numbers for you.

It should work if you both have AT&T(or the saw carrier)