In order to resolve a "Call Forwarding" glitch on my 5S - Apple has suggested I try swapping the T-Mobile nano-sim card (now activated) which came with the phone - for a new T-Mobile card which I'd get at local T-Mobile store. My phone was purchased thru the Apple Online Store, contract-free. It arrived with the T-Mobile card pre-installed. Might this swap cause my "unlocked" phone to become a "locked" device? ie: What's the connection (if any) between a t-Mobile nano sim supplied with an iPhone purchased directly thru Apple - and a card obtained at a T-Mobile store?