call blocking 25 numbers?

When in voice, call history, I want to block a caller...

Click on options, block caller, then the number with a list of other numbers pops up.

I do not know what those other numbers are? Are they numbers I have already blocked?

 If not, and I can only block 25 callers, does this not eat up that 25 numbers?

 Is there a way to block only one caller at a time? (if those other numbers are not ones I have blocked already) ?

 If they are numbers I have blocked in the past, can I delete the entire list?

Thanks