If they left a message stating who they were and or why they called, there wouldn't be so many questions on who this person is. I don't answer numbers that aren't saved in my phone, but I do call back if a message is left. I'm curious as to why it would be their process to not leave anything indication that it was them calling.
Legitimate call from the US Postal Service customer service team about a misrouted and delayed Priority Mail Express (overnight) package for which I had submitted a claim. This was not a spam or telemarketer call. They had a legitimate reason to contact me.