When you dial an 8-digit phone number, the number you set here will be automatically prefixed to enable local dialling.
If this is not set, then the system will take the area code from your caller ID and use this instead.
If this is not set and your caller ID is set to private you may hear an error when dialling local numbers stating "Please dial a 10 digit phone number". To resolve this problem please set the Local Area Dialling Prefix on the Device in VoIP Commander.