Sorry, but I won't answer any number that lacks a caller ID. Sometimes I will have someone say to me, "You never pick up your phone!" I will say, "did you leave me a message?", which usually gets answered with, "Um....no."
If you don't leave a message, how the heck am I going to know who you are or what you want? I do return any message I get. There are too many spam callers out there for me to answer a number I don't recognize.