I just received a collection bill from att&the mobility

Stating that I owe 773.03. The collection company name is: Credence Resource Management LLC and it claims that att&t mobility is owed 773.03
I'm currently ano att&the customer, you guys withdraw from my account monthly and I have always stayed current. I also have had the same telephone number for many years, more than 10 and I don't understand what's going on. Please let me know what you suggest since this company is representing themselves as you guys, or is this company part of att&t wireless?

Thanks,

Concerned client.