This is a junk debt buyer trying to frighten you into contacting them and making payment arrangements, thus resetting the SOL and their ability to actually sue.

Process servers don't call you to tell you that they're coming. They just show up. Calling ahead would defeat the purpose of personal service, as the intended recipient could simply decide to not be home. There is no law requiring you to wait around for a process server to show up. A suit cannot proceed until service is perfected.