Friend's dad has a sim unlocked 1.1.1 iphone. He travels alot, and uses it on other GSM networks. He would however like to start using visual voice mail, and take advantage of the cheaper iphone plans than the regular/data AT&T plan that he's on now.

What are the steps/order to telling AT&T that he has an iphone, switching to an iphone plan - and keeping the phone sim unlocked for traveling in the future...

Thanks,