I bought an iPhone 5s last year from the Three Store which came unlocked to use any network. Just recently I took the Three SIM card out and put in a sim from EE. Now when I go into my EE account and go to my mobile phone in my account it tells me that my iPhone is now locked to the EE network and can only use an EE sim now on my iPhone. This is very strange considering I bought my iPhone 5s unlocked to use any sim. Please advise.