Vodaphone basic smartphone locks again 3 months after successful unlocking

[Android Central Question]

Bought Vodaphone basic smartphone for non-tekkie wife for emergency use on Three PAYG. Seller provided unlock code. After 3 months and £3 worth of calls she found phone locked. I could unlock it OK BUT if that had happened when she was alone in an emergency situation would have been disastrous. I have written the unlock code on the phone BUT since she WILL panic I would prefer to know why this happened (as it's impossible, Vodaphone tells me) and, more importantly, that this will not happen again. Thank you.

 

[B. Diddy]

Welcome to Android Central! Have you checked with Vodafone to see if they're somehow re-locking the phone because it doesn't meet their unlock policy criteria?