This is a scam. Real process servers don't call in advance. Nor do they tell you that you can pay your way out of being served. Block and ignore their calls.
No one was served papers in connection with calls from this number. They are just thieves using some vague threat of being served in the hopes that you will pay them, to prevent being served.
Thanks everyone. They are so convincing. I had a payday loan. It was so long ago. I don't remember owing but they made me really have to think. I'm a little nervous...