It was reported that Quest Diagnostics Corporation was hacked in Dec 2016 and 35 thousand patient records were copied by unknown hackers. It appears that these supposed collection calls come from Pakistan and they have info about patients billing. They call persistently but never leave a message, if you do answer, they try to get your credit card number by quoting you some of your personal info and your test dates and billing amounts. Even if the bill was paid to Quest, they deny and try to get you to give credit card info over the phone. Do not answer these calls to your phone, and certainly do not give them any payments or any more personal info. They apparently have the last 4 digits of your social, but not the whole number.