Someone name Brandon called my grandmother. Only left his name and this number! He did not leave a message

Someone named Greg called my work.  Not sure who or why.

Who is this

I got a call from this number as well.  They left a message saying they are a debt collection service.  Didn't say who they were looking for, just gave a "claim number" for when you call back.
I'd ignore this it's probably one of those companies that buy old debt and if you acknowledge it in any way it can re-start the statute of limitations.