They call me fairly often, and they try to get me to "verify my SSN or DOB" without telling me who they are, giving me my mini-Miranda, saying who they were collecting for, and I tell them, EVERY TIME, that I will not give any of that info to someone who is calling me. I told them to stop calling me, and that I'll only correspond with them via US Mail. I've asked for verification of the debt, as well, and it's been months, and I have heard nothing from anyone. I don't owe any medical bills, I have excellent insurance (that I pay $300+/month for) and I pay my copays at every visit. I don't owe a penny.
Call and never leave message. Finally called them and still don't know what they are, but a very rude person named Kara pulled a name and number off caller ID and said it would be removed from whatever list she wouldn't answer my questions about.