you can't change the 'default dial peer' as Dial peer 0 is hard coded into IOS. You can nominate another dial peer to act as the default dial peer using the system cli -
dial-peer voice 1111 voip system
incoming called-number .
codec g711ulaw
dtmf-relay h245-alphanumeric
This allows you to change the default characteristics of the voice calls as specified on the configured dial peer.
