I have no idea who is calling; they did not leave a message. I am wondering if this is a computer generated call from a collection agency.

It wouldnt be a collection agency because I got one too but didnt answer it.   I do not owe money so it cant be a collection agency.   I am guessing it is a telemarketer.    If you answer and there is a delay before you hear anything, it is an automatic dialer type  which usually means telemarketer