Does anyone know the updated answer to this?: I'm currently on StraightTalk and I'm traveling to the UK soon and already have a SIM from "Three" that I've used there before with my iPhone 6. I've checked on that SIM and it's still operational. I also have an old, unlocked iPhone 3GS. I'm wondering about getting both phone numbers registered with iMessage so that the iMessage experience will be seamless. Do I have to take the StraightTalk SIM out of the iPhone 6 and put it in the iPhone 3GS (with SIM adapter) before I leave the USA...or...can I do the swap when I'm in the UK and on wifi? Thanks in advance.