If the phone is not blacklisted then a new EE sim will work, or any sim from any EE network company (orange, Virgin etc.). Other true networks, ie. Vodafone, 3, O2 etc. (and sub companies that use their networks), won't work. EE will unlock, for a fee. You may well get a cheaper fee on the high street from mobile phone repair shops. It is easy and cheap. DIY is possible, but you would need to buy phone unblocking kit which would be expensive unless you're setting up your own unlocking service. EE or the shops is your best plan. V. cheap, v. easy.
Oh, and of course it is legal. She owns the phone, it's not vandalism if you mod your own stuff. Who else could possibly have a say?
01-22-2015 08:47 AM