Great to hear that you already have the iPhone in-hand. I'd first suggest getting that phone unlocked if it isn't already (AT&T should do it for free) and then bringing that phone to a T-Mobile retail store to get set up with a working SIM card.
This will be perfect since texting is your main concern, your ability to talk and text will be there right away, data speeds will be slower than the iPhone advertises however. T-Mobile has a few different plans that can group all these services for you at a very competitive price. Feel free to private message me if you would like to go over rate plans that will work well with your phone.
I hope this helps,
I was in the same boat as you. I came over from ATT. If you are/leave/left ATT in good standing (all bills paid), you
can request your ATT iphone be unlocked:
They do it FREE of charge. YES, you will have to get a sim card from Tmobile. I bought
a prepaid one like this:
And used it on my 4S on the $30 month 4G plan - http://prepaid-phones.t-mobile.com/monthly-4g-plans
YES, you will be able to talk, text and use Internet on your iPhone with this prepaid plan.
Oh, and if you have problems with your iPhone on Tmobile - these discussion boards, the people and Mods who
post are really helpfull.
I hope everything works out for you.
I will help you,
In order to put T-mobile sim card on your AT&T locked iPhone 4,You will need to unlock it first.You can unlock your iPhone 4 using remote unlocking method.It's one of the best way to unlock your iPhone.If you are interested in this method visit Mobile-unlocker.com Here you can unlock your mobile phone at low cost.After unlocking you will be able to use all features.