Can't add Zaptel extension

I am using ZAP/1 ZAP/2, etc for the DAHDI FXO port channels, but for some reason they don’t work. If I enter ZAP/1, etc, when calling the other extension, I will be send to the voice mail right away. If I leave the “dial” extension empty, it will generate busy tone immediately.

When I have zap/1, zap/2 … in the dial field, the asterisk log file showing ZAP not found.