Letter of final demand 1 year six months later?

So Cell C sends me a Final letter of demand one and half years after I transferred it to my ex-wife. When transferring the contract which had ended. And jumping through all the hoops signing every document they could possibly throw at me and getting confirmation that the account was up to date nothing owed. A year and half later they send a letter of final demand. But the joke doesn't stop there nope. The email address they say I need to contact is rejected by the postmaster. The help line I called says because I am no longer the registered owner of the account they cant help me. What a bunch of useless idiots.

So Cell C contact me via here or by my email which you have.