I already have a standard cell plan with T-Mobile for my Razr, but when I got my Neo, I was able to walk into the local T-Mobile store and just pick up a prepaid SIM card plan for $15. You just tell them (or AT&T) that you already have a phone, and you just want the service plan, and they'll give it to you. T-Mobile offers both monthly contract-free and prepaid plans without the need to purchase a phone, so you should be perfectly fine.