My answer button didn't work either, so I uninstalled the stock call handling app. I then installed call handling pro, it sucked Cuz there was too much lag between someone calling and it showing up on the watch, but the answer button worked. I uninstalled call handling pro, and reinstalled the stock call handling app, and like magic there is an answer button now and it works.
Device: Samsung Galaxy S Telus Fascinate 3G+ (SGH-T959D)
Firmware: Android 4.1.2
Rom: AOKP JB M1
Device: Sony Smartwatch 2 (SW2)