Callfile:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~``
Channel: Local/$phone_number@fxs
MaxRetries: 10
# Retry in 5 min
RetryTime: 300
WaitTime: 45

Context: voicemail_callback
Extension: s
Priority: 1
SetVar: mailbox=$mailbox

[fxs]
exten => _NXXNXXXXXX,1,Dial(Zap/g1/${EXTEN},,g)
exten => _NXXNXXXXXX,2,Macro(handle-hangup)

[voicemail_callback]
exten => s,1,Wait(3)
exten => s,2,SendDTMF(${mailbox})
exten => s,3,Hangup