Cox Communications Billing For Decaessed Mother?

My fathers mother had cox cabel (TV not internet) at address A – and when she passed away we forgot to cancel her TV for about 2 months (( Funeral arrangements grief ect… ))
Now for the past 5 years we have had an account at address B with cox High speed internet. A few months after my fathers mother passed away, we thought it would be best to move in to her old house (( Address A ))
We called cox and asked “How much is it to move our plan from address B to address A” Cox happily replied “No charge”
>>And so they moved the plan/instaled the high speed at address A
4 days later we get a phone call and a bill saying “Your account balance is now due $600+ blah blah charges”
A few angry phone calls and they claim we owe payment for the dead persons bills because we are now at address A and because my fathers last name is the same as his mothers (( wtf? ))
So now we have no internet or anything :/
is this legal? Or do i just need to complain a few times?