I had a similar problem with Mint, it took 5 months for them to get the message, they even sent 2 seperate debt collecting agencies after me, once I had convinced the first one that I was in an IVA a second company were instructed to come after me, these people are very persistent, don't believe a word you tell them and can become very threatening. At the end of the day they can't do anything to you as long as you keep to the terms of your IVA, I know it's hard but tell them very politely you are in an IVA and they are not allowed to demand payments off you, then send all their details to your IP and let him handle it, that's what he is being paid for.
Be patient they will eventually get the message.
IVA COMPLETED ON THE 17th MARCH, FINAL I&E COMPLETED 26th APRIL, COMPLETION CERTIFICATE ARRIVED 2nd AUGUST