My question involves collection proceedings in the State of: california

My home was robbed while I was away many years ago. They stole a lot of things including old documents including banking and such. Months later I receive a collection call from Sprint saying I owe them $1500 in cell phone usage. I have never used Sprint, have always and still use Verizon. I explain to them I was robbed and the thieves probably opened a cell phone using my name. I have gone to police after robbery happened and proceeded with identity theft precautions but the Sprint cell phone was never caught in time I guess. I continued to ignore the Sprint people. Years have passed, have continued to check Credit Report and the Sprint cell phone is and never was on my credit reports. But i get a piece of letter every couple of months from a collection law firm representing sprint to collect the debt. But I've continued to ignore it and now the debt collection letter is reduced every letter I get and yet nothing on credit reports.

My question is should I still ignore this? I have talked to Sprint when the incident first happened and talked to probably 10 different people and had them realize this is not my account that I had opened and was a result of identity theft.