Someone

[default]
exten => 1000,1,Dial,Zap/1|3
;exten => 1000,3,Hangup
exten => 1000,102,Voicemail,b1000
;exten => 1000,103,Hangup
exten => _NXXXXXX,1,Dial,Zap/4|20 ; Any 7-digit extension that doesnt start with 1
[incoming]
;exten => s,1,Wait(1)
exten => s,2,Dial(Zap/1|20,t) ; rings the phone plugged into the FXS port
exten => s,3,Voicemail,u1000
exten => s,4,Hangup
exten => s,103,Voicemail,b1000
exten => s,104,Hangup
exten => s,1,Dial(Zap/1/${EXTEN:1})