Most likely this group:

CMRE Financial Services   (Appears to specialize in medical bill collection--a lot of complaints about their procedures).

http://www.cmrefsi.com/

800-783-9118 or 714-528-3200
By fax:
714-528-5863
By email:
customerservice@cmrefsi.com
By mail:
3075 E. Imperial Hwy, Suite 200
Brea, CA 92821

________________________________
THIS BUSINESS IS NOT BBB ACCREDITED
CMRE Financial Services Inc
Alternate Business Names
 Florida Emergency Physicians
 Physicians Business Bureau
 Radiology Associates of Nevada

Phone: (800) 783-9118
Fax: (714) 528-5863
View Additional Phone Numbers
(714) 582-3200(Phone)
3075 E Imperial Hwy #200, Brea, CA 92821
customerservice@cmrefsi.com
http://www.cmrefsi.com
BBB® B Rating
On a scale of A+ to F
148 Complaints in the last three years according to BBB!
________________________

Just because someone on the phone calling you says you owe a debt does not mean you actually owe it.

If a voice on the telephone claims you owe a debt:

1.    Demand the person give you his or her name, company name, and address
2.     Demand a debt validation letter by US mail.  E-mail no good.
3.    Do not pay  the collection agent or anyone else a  dime until the debt is validated

Of course they won’t do this, no scammer ever will, so simply ignore them and block their calls.

If they are bothering you, report them:

http://www.consumer.ftc.gov/articles/0149-debt-collection (Threatening civil proceedings with no intent to follow through, or threatening criminal process in order to collect a civil debt is a  violation of the Federal Fair Debt Collection Practices Act.)  The same goes for endless telephone harassment

Also contact your local attorney general