lg g3 answer button doesn't work when called from an "unknown" number

[goodwrenchinId]

On a regular call apparently you have to hold onto the answer button then slide it to get it to answer but I haven't been able to answer this person trying to call me that is unknown. A little box pops up and asks if you want to block this call but didn't see any way of answering. the green answer button wouldn't work. What gives?