I'm very angry at CoServ. After we sold our house...

I'm very angry at CoServ. After we sold our house in Texas, apparently there was a remaining balance. For 18 months, NOT ONE PERSON from CoServ has called to try and let us know or send me a bill (they obviously know my cell phone because the collection agency that they sent my debt to knew it!!). I called their customer service and NOT ONE PERSON has called me back. How can I pay a bill if I do not know about it!!!! Really bad customer service.