Will my SIM card work for an iPhone?

I have a smaller AT&T phone from a while ago. I want to take out the SIM card and put it into my iPhone 6. Will this work?
*Best answers will be given*

It helps to know the make and model of the phone, not the carrier.

If the smaller AT&T phone uses the Nano SIM, then yes. But you didn't tell us what other AT&T phone you have. Not all phones use the same type of SIM card size.

If the smaller phone can accept nanoSIMs yea. More than likely though you'd have to use an adapter to get it to fit securely into the phone.