Spam caller, hung up, didn't leave message, callback results in 'press 9 to be added to our DO NOT CALL list'.

Seems spam, callback did indeed result in 'press 9 to be added to our DO NOT CALL list'.    I didn't press 9.

Get calls at my office number multiple times per day.  Never leave a message.  Assuming it's a SPAM call since, if you   call the number as I did from another phone, you just get a message to press 9 to add number to their Do Not Call list which I know never to do.  I've been told this is a way spammers will know for sure that my number is a real number of the millions of numbers they are attempting to reach every day.   More annoying!