[get_msg_count]
exten => s,1,Verbose(1|Getting number of messages in the mailbox)
exten => s,n,Set(X=0000)
exten => s,n,While($[${STAT(e,${MSG_PATH}/msg${X}.txt)}])
exten => s,n,Verbose(1|Current message count: ${X})
exten => s,n,Set(X=$[${X} + 1])
exten => s,n,Set(X=${IF($[${LEN(${X})} = 1]?000:${IF($[${LEN(${X}(} = 2]?00:${IF($[${LEN(${X})} = 3]?0)})})}${X})

exten => s,n,NoOp(${X})
exten => s,n,EndWhile()

[voicemail]
exten => 7,1,Answer()
exten => 7,n,Set(VM_CTX=default)
exten => 7,n,Set(MAILBOX=100)
exten => 7,n,Set(FOLDER=INBOX)
exten => 7,n,Set(MSGNUM=0000)
exten => 7,n,Set(MSG_PATH=${HOME}/${VM_CTX}/${MAILBOX}/${FOLDER})
exten => 7,n,GoSub(get_msg_count,s,1())