You can configure telephony settings for the Vocera Voice Server and Vocera SIP Telephony Gateway in the Administration Console. See Configuring Telephony.
For advanced configuration of Vocera SIP Telephony Gateway and Vocera Client Gateway, you can edit a properties file called vgwproperties.txt. You can use any text editor to modify the file. If you make any changes, you must restart the Vocera SIP Telephony Gateway or Vocera Client Gateway to load the properties into memory.