Transferring Number on PAYG?

[hooneeawr]

It's much easier these days to transfer your number on a contract. What is it like with PAYG?
I ask, as my mum is on T-Mobile but I fancy getting her a motorola W220 from phones4u. Only trouble is it is on Vodaphone and the phone apparently is a pain to unlock.

 

[Hot Sponge]

I would have thought that it's a similar process as a contract number port.