What is dial-peer 0?

dial-peer 0 is a default dial-peer for inbound call when no other specific dial-peer is matched.

There are several parameters used to match a inbound call within dial-peer configuration mode. They are matched in the following order.

1. incoming-called number (DNIS:Dialed Number Identification Service)
2. answer-address (ANI:Automatic Number Identification)
3. destination-pattern (ANI)
4. port (For inbound POTS call only)
5. dial-peer 0

Dial-peer 0 will be matched automatically if all the above 4 parameters are not exist.

Once dial-peer is matched the following “default condition” also applied and not tunable.

• accept any voice codec
• NO DTMF relay
• Voice Packets marked as IP precedence 0
• VAD enabled
• NO RSVP support
• Fax-rate voice: The router will limit the bandwidth available to fax signals to the
  maximum allowed by the VoIP codec. This could devastate fax calls if you are using
  a low-bandwidth compressed codec.
• No application support: Dial peer 0 cannot refer calls to outside applications, such
  as an Interactive Voice Response (IVR) system.
• NO DID support

As dial-peer bring some bad effect for voice call, configure your dial-peer to avoid it as possible as you can.