There are some caveats. First they can't bill your number for anything. So any service that would need to bill your phone won't work. (say accept a collect call, call a phone sex line, etc) You will not be able to make international calls unless specifically in your plan. If you take a trip to Canada you'll be out of luck.
You will not roam. This is generally not an issue, but if you are on ATT and find yourself roaming already you may have an issue. NO LTE. All prepaids get lower priority on towers from what I understand. Normally not an issue, but if you're trying to make a call on an overloaded tower I hear they take care of post-paid first.
I haven't run into issues, but this is what I've heard about limitations. Most of this applies to any pre-paid carrier.
I've had straight for about 5 months now with ATT sim...
- no LTE
- can make international calls - I have made calls into canada just in the last week
- roaming is goofy. Went out into WI (I live in twin cities area), and coverage was in and out... no data roaming, but could make calls.
- MMS needs to be setup. it is kind of quirky and will require special settings. I have made matters even worse by using google voice which doesn't really support MMS.
- don't expect any customer support. had a local issue with ATT towers. ST didn't have a clue. their suggested fix was to send out new sim cards. ended up a tower issue and it affected all ATT users. had to wait it out till ATT fixed the towers. When you have issues, be prepared to deal with them on your own.