Block call on Apple Watch via ATT NumberSync

My apple watch keeps getting calls from a spammer. These calls appear only on the watch, not on the phone, which makes me suspect that they are probably being direct dialed to the phone number that the watch itself has. (On ATT, your apple watch gets a separate phone number, and ATT joins these two together using "NumberSync").

The problem seems to be that if a caller dials the watch's phone# directly, the call goes to the watch, and there's no way to then block the caller. All other calls come in to the watch and show up on the phone, but not this one.

These calls are from a known spam / scam caller and are coming in 3-5x a day.

Help!

p.s. I see that ATT does give you an option to "Change your wireless number" that applies to the Apple Watch, but it's $36 and it's not clear this would actually fix the problem.