He said to me, "You can't listen to the vernacular." How do I answer the phone?

If you can understand and speak the vernacular, say it in the vernacular-no, just understand (then don't confuse it).

If you can understand the vernacular but can't speak it, you can say-Hey, I (deny that you can't speak it).

If you can't understand or speak the vernacular, just say-hey (you can't speak the vernacular if you agree).