I would speak to them, not giving away any information to see which of your creditors is involved and whether they have bought the debt or are attempting collection.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014
Their site states it is "your" responsibility to contact them.
In the absence of any written information about who they are acting on behalf of I would block the number on your phone.
IVA started March 2011, Completed March 2016 and certificate issued 11 days after final payment. It was not always easy but then some of the best decisions aren't.
Again, if you do speak to them, tell them nothing until they have satisfied you that they know enough to be legit. Even then only give them the IP's details and ask that they speak to the IP only.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014