Call centres haven't a clue what is going on. I would ignore the calls and send any letters to Mel. They will sort it.
If you choose to answer the calls then you can refuse to confirm the security questions - they can't continue with the calls after that.
If you have online stuff with your creditors you can go in now and put a totally fictional number in.
You can always change your phone number.
Sharing from experiences of dealing with debt
The greatness of a man is not in how much wealth he acquires, but in his integrity and his ability to affect those around him positively.
Bob Marley. http://kallis3.blogs.iva.co.uk