Standard extensions
- a: Called when user presses '*' during a voicemail greeting
- h: Hangup extension
- i: invalid extension
- o: Operator extension, used for operator exit by pressing zero in voicemail
- s: Start extension in context
- t: Timeout extension
- T: AbsoluteTimeout() extension
- failed: used if an auto-dial out call fails (that had context, priority and extension specified)
- fax: used for fax detection on Zap channels
- talk: used in conjuction with BackgroundDetect
