I've had the Active 2 since January. Today when I went to answer a call on the watch, I got a "check your phone" directing me to finish answering the call on my phone. Answering/placing calls on the watch itself worked fine last I used it. I checked GoodNight, Do Not Disturb, Theatre Mode. Restarted the watch and phone, repaired, checked for updates, etc. Anything I'm missing here? The ability to do that was one of the reasons I purchased.
I'm guessing but the next I would check is that the permission to "Telephone" is set both in watch "Telephone" and all your phones related apps (Galaxy Wearable, Watch plugin).
(Perhaps it needs toggling or cache clearing if something is messed up.)