I've been with AT&T for about 13 years but recently their service in my (mostly rural) area tanked. It used to be decent but now it's impossible to stay on a phone call and LTE constantly switches to 4G, or Edge, then No Service. It's not a phone issue -- everyone in my family has an AT&T phone and we've all had problems. I'm considering ditching AT&T and switching to something prepaid, either Straight Talk or Cricket. I know Cricket is still going to be the same crap service, but at least I won't be paying ~$350 a month for it. I'd like to start out by testing a few of the prepaid services before I cancel my AT&T account and port my number. Is it possible to get a new SIM from, say, Straight Talk or Cricket, and test it out using my unlocked iPhone? Or will putting a new SIM in the phone somehow invalidate/remove it from AT&T's network and cause my current SIM to stop working? Will activating my phone's IMEI with another carrier disrupt my AT&T service? Thanks.