It shouldn't make any difference Nicola. Once they are told that you are in an IVA, then all calls, letters etc should cease. Ok, it may take a few weeks for the details to filter down, but once you have told them, that should stop them.
I was plagued with calls from Mercers, but I never answered them until the day my IVA was agreed - 21st April, but once I explained things to them they were fine and I have not been bothered since.
As Andy says, they cannot do anything now as you are protected by your IVA. I would suggest that you either don't answer the phone to them, or just tell them that you are in an IVA, they shouldn't be calling anymore and tell them you will report them for harrassment. Make sure you get a name as well. I would also refer them to your IP. Then I'd put the phone down before entering into anymore conversation with them.
They're just trying to get you to pay money, which you can't do as it could fail your IVA.
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