如果要用 Asterisk 做 IVR 的話，就要寫 Extern 語法，以下紀錄一下用法。

i = invalid, jump here when an invalid number is dialed.
t = timeout, jump here when we timeout.
h = hangup, jump here when a call in hung up.
s = standard, jump here if the dialed extension is unknown (used for generic processing).
o = operator, a special extension to escape voice mail (configurable)