WebAfrica demanding money from someone who passed away

My father-in-law had an internet contract with WebAfrica prior to his passing. After he passed away we attempted to get this contract cancelled for a number of months via their WhatsApp number without success. They kept telling me we had to pay another month's installment in order for the contract to be cancelled, which is preposterous. Eventually I called their sales department, as this is the only way a person can speak to a human being, and emailed a lady all of the relevant information pertaining to my father-in-law's passing and was advised that the matter was now resolved. Only to find out that this is not the case, and we are receiving letters from their legal department demanding that we pay the month's outstanding amount. I have emailed the legal department all of the same documentation pertaining to my father-in-law's passing, but still receive the letters of demand????? I have once again been calling their sales department and have left messages requesting a call back without success. How do we get this matter resolved????