Cancel contract after death

My father passed more than a year ago. We contacted cellc and let them know, and asked them to cancel the contract. They asked for documentation (i.e. death certificate) that we provided, and asked for a number were we could be reached, that we also provided. They never rep**** to the email, or called the number. We sent it though multiple times. Nothing has ever been done, and the contract has not been cancelled. Zero communication from cellc. They simply do not respond to emails. They have now handed the case over to a collection agency. What now? Are they going to try and collect from my dead father? How do you get cellc to communicate?