I have ST. Great service. Around here they use ATT or Verizon towers depending on your service. Forget about customer service over the phone. You get Central Americans (English is the second language) with no brains. If they come to the USA they will be qualified to work for TSA!
I use Facebook to get customer service and that works. Each sim is matched to a phone. So they will move your # to the sim in the new phone. Do that via FB trust me.