I am handing down an iPhone 4S and they are currently on a pre-paid T-Mobile plan. They only use the phone for emergencies and to give you an idea, on a 1000 minute plan that costs $100 a year, they used maybe 30 mins last year. Still cheaper than spending $30-$40 a month. My 4S is an AT&T unlocked model so it will run on T-Mobile. They only want the phone for calls and to take pictures and videos. They have absolutely no need for Internet. What is my best option? Can I simply get a T-Mobile SIM and transfer the number over from their phone to the iPhone and keep the $100 plan? I am aware of the $30 a month plan that includes unlimited data but they have no use for that.