Please note, this is a scam! A process server will NEVER call you. They just show up. These people threaten to show up to your home or place of employment, which is a violation of the FDCPA. They commit numerous other FDCPA (http://www.nolo.com/legal-encyclopedia/damages-fdcpa-violations.html) violations and you are entitled to sue them, at $1,000 for each violation.
They use lots of different numbers and lots of different names. One of these losers used the name Jack Smith (called from a blocked number but left 877-589-2645). The idiot called twice, looking for different people. Each time he stated he was from a different county in different states. Wow, he must have supersonic travel to get from one end of the country to another "between 3-5 pm" on the same day!
The greatest weapon a scammer has is the telephone. Take it away. Don't answer calls from unknown numbers. Don't agree to send ANY money - ever! If you already have, report the credit/debit card stolen. Report all harassment to the FTC (https://www.ftc.gov/about-ftc/bureaus-offices/bureau-consumer-protection), the CFPB (before Republicans gut the agency, https://www.consumerfinance.gov/) or your state attorney general or consumer protection office (https://www.usa.gov/state-consumer).