What the procedure for dept before hand over to third party.

I once Hired an Avis car, unfortunate I was involved in an incident, I pay some of account charged beyond that and at a later stage I failed to continue with payment. now Today I am being called by person from CWX telling me my account has been handed over, why only contact me when my account has been handed over and not before that happens, is this the way they make money out of people, they got my contact details from cell number and email, how do you guarantee that sms you send me reached or if it did I read it. I owe avis and they should have communicated with me about this before they rush to milk more money out of this, CWX must return my account to avis at no cost and avis should in future consult their client before they hand over account so there can be some arrangement after hearing what made customer to fail paying.