This number rang my cell 3 times in succession, let it ring once, and then hung up. When I tried calling it back, it said the number was no longer valid.

I have experienced  the same thing.   I tried to call the number back only to be informed that the number is not in use.   I have an (app) for that!  I will add it to my little blacklist.

received call this morning [cell phone]...i did not say anything until someone came on the line...I said hello, they asked "Is this (my first name)?" I said yes, hello...and they hung up.  When I tried to call back says this number not in service.  This is the second call of this type [different number] in the last two weeks...no response, not a working number...other one was NOT in my area code like this one.  Very annoying.

They keep calling me. I called the number to see who it is and the recording says the number is not in service. Hmmmmmmmmm! It's election time. I'm getting calls nationally as well. It's healthcare soliciting time too.