Well, they could stop you if they really wanted; they could IMEI sniff. AT&T does it to detect "smartphones" so they can helpfully add 'mandatory services' to your plan without your consent - it'd be easy for T-Mobile to block Apple's IMEI ranges from registering on the network regardless of whether your SIM worked or not.
The FCC should have implemented some actual common carrier regulations/a Carterfone-equivalent for the mobile industry. Instead, they're so mired in regulatory capture we get nothing.
The FCC should have implemented some actual common carrier regulations/a Carterfone-equivalent for the mobile industry. Instead, they're so mired in regulatory capture we get nothing.