For some reason, land-line providers make call blocking of particular numbers difficult and expensive. But call blocking of any number(s) you want to block is simple on many cellphones. Although the specifics are different depending on what phone you own, generically, here is what to do:
1. Go to your Phone Call Settings.
2. Go to Auto Reject List and add the number(s) you want to automatically reject.
And you will see other options such as being able to send a rejection message, or being able to set a schedule for when blocking mode is on (which might be particularly useful if you want to block all calls (which you can do) while leaving a message for the caller(s) when you're say doing your marathon or tri-athalete training).