Called me, rang once, no message

Just called. One ring, no msg. They want a call back so they know they have a good number for later calls.

Called and  rang once and hung up.  Must be a faulty robo-caller.

Called, rang once, hung up. Blocked them!

I have a little fun with them.  I have VOIPO, inexpensive, but it lets me block several ways. Since they won't stop, I just forward their number right back at them and in some cases it causes their system to go into a Loop. Cool.
See SEOscam.net.  Voipo at FlatRateCalling.net