Note: The exclamation mark wildcard, which is available only in Asterisk 1.2 and later, behaves specially it will match as soon as can without waiting for the dialling to complete, but it will not match until it is unambiguous, and the number being dialled cannot match any other extension in the context. It was designed for use as follows, so that as soon as the digits dialled don't match '001800...' the outgoing telephone line will be picked up and overlap dialling will be used (with full audio feedback from 'earlyb3' etc.)
Context "outgoing":
Extension Description
_001800NXXXXXX Free US calls made by VoIP
_X! Outgoing calls via normal telco, with overlap dial.