Having trouble with transferring number

OK, please stay with me on the history!  I am losing the will to live with this!  My mum has had an ID Mobile contract for at least 3 years since my late dad set it up for her.  She cannot use a computer so had no idea how to access her account.  She recently changed her bank account but had no way of informing ID mobile that it had changed.  I tried to contact someone at ID mobile for her but ended in a perpetual loop of suggested answers that didnt help.  I was forced to start a contract with another network with the plan to transfer her number.  When the SIM arrived we found that her phone was locked.  She put the old SIM card in a safe place = she couldnt find it again.  I spent a couple of hours on chats with ID mobile and Carphone Warehouse only to learn that they couldn't do anything to unlock the phone.  I thought that the simplest solution to get her phone working again would be to cancel the new card on the other network and start a new ID mobile contract and transfer her old number to the new SIM card using the PAC code that I got at the start of the process.  I’ve just tried to do this final step and get the message back that I cant do ID mobile to ID mobile.  This seems crazy.  Shouldnt it be easier to transfer numbers on the same provider?!?!  Can anybody help as we have no way of accessing the old account but she needs to keep the number as her life’s contacts all have that one.  Please help!!!!