Change of default values in 'Add SIP Extension'

hello,

i like to change the default values of some drop down boxes in the ‘Add SIP Extension’ form. i like to change the following:

Call Waiting from Enable to Disable
Record Incoming from On Demand to Never
Record Outgoing from On Demand to Never
and in the voicemail section:
Status from Disabled to Enabled
Voicemail Password having a default in the field of ‘0000’

where can i make the changes ? Any help is very much appreciated.

kind regards
Jan