Asked to pay an account for a person who passed.

When my mom passed away i went to telkom east gate and was told they could move my late moms number to my dad. I gave them my late moms death certificates etc and asked them if they would make it a nil balance because my mom passed away and my dad cannot afford the amount. Now i have been pestered every week by p&p attorneys to pay that balance after i was told it would be voided