I thought the phone was yours to keep after the contract was up, just got this from t-mobile:
"If you want to carry on using the same phone, we'll also need to give you a subsidy PIN.
We can give you this for a fee of £15 inc VAT, but you will have to check with your new network if your phone will still work."
I didn't think they could charge you if you want to carry on using the phone?
Only problem is that I sold the phone, don't want an angry buyer with a blocked phone!