Good luck with the phone call.
I would recommend that if anyone is likely to have a stressful phonecall to plan the call and write down the points you want to cover.
you need to take control and manage the conversation and certainly don't get into a conversation.
I would suggest something like this.
Good morning - my name is XXX. Your Mr Y came to see me on 7th january at which time he recommended that I exit my IVA and go B. He also suggested that I had been missold an IVA.
I have today spoken with my IVA supervisor - MR ... and we have come to an agreement that I am able to fulfil. I therefore believe that I have indeed not been missold an IVA and as such I wish to cancel your contract. As you have not represented me with any of my creditors or in court I would appreciate a full refund and expect this within 5 days.
If I do not receive a full refund I will have no option but to inform the Office of fair trading and make a formal complain of misselling.
Hope the above helps a little.
Office of Fair Trading web site
http://www.oft.gov.uk/
To be fair to them if they have actually done some work it miht be appropriate for them to retain some funds but they would need to clearly identify what this was and it would need to be justified.