Any tips for if they phone again? Simply put the phone down? Stick on some holding music?!

Hello all, New to the forum, hoping for some advice on this issue. I too received a "fine" from Euro Car Parks for overstaying at a Sainsbury's car park in south London last year and, as per all the advice on here, have ignored all letters (which now number five). However, I have now received a call from Euro Car Parks. I was slightly on the back foot and said I hadn't received any letters (supposing that they can't prove I have). They are now sending another letter "with any evidence that we have". Has anyone heard of ECP actually calling people before? I'm wondering if now is the time to respond with the templated letter? Thanks all