1) Log in to the Sonic member portal
2) Select "voice" from the left hand column
3) Select your phone number in the middle of the display
4) Under "advanced settings" select "Reject Selected"
5) In the small text that appears immediately beneath that, click "rejection list"
6) Enter the numbers you want to block, one at a time
Note that this won't work well if the caller you want to block is changing their number every day.