Can I use an AT&T or T-Mobile iPhone with Family Mobile?

I understand that Family Mobile is powered by T-Mobile so I was wondering if I buy an unlocked AT&T or T-Mobile iPhone can I use it with my Family Mobile sim card? I just would like a straight answer before I actually go out and buy the phone haha.

Yes, you can switch lines if your mobile device is unlocked!

Yes, you can use any carrier you want as far as your iPhone is unlocked and not blacklisted! T-Mobile and AT&T are both GSM networks, so no problem at all.