are you there?
Some have some mechanism to block the number regardless of people being called have caller ID. On the other end of that some people have that to show the number regardless of if the number was blocked.
Obviously caller ID info is not being transmitted
They have used an option in in their phone
that keeps their number from being displayed on other phones.
Landline callers can do it using *67.
Dial this sequence: *67that-bad-number# for example: *1237894560#
When they answer, you may ask them your question, "Why". If it is a solicitor or other bothersome caller and you wish they'd never call again, sign up at "nomorobo" dot com and report the offending numbers that bother you. "Nomorobo" will net them for you. After that,, answer all calls on or after the second ring. The service is free and i've been using it for a couple years now.