You don't want specific phones to dial outside from softphone?
Admin --> Phones --> Select the one you want to restrict --> Phone Context --> change "default" to "vicidial-auto-phones"
Wait some minutes and make sure the changed config files were generated / updated. Type in asterisk cli : 1. "dialplan reload" and 2. "sip reload"
Phone Context -This is the dial plan context that this phone will use to dial out. If you are running a call center and you do not want your agents to be able to dial out outside of the agent screen applicaiton for example, then you would set this field to a dialplan context that does not exist, something like agent-nodial. default is default.
Vicibox 6.0.2 from Vicibox_v.6.0.x86_64-6.0.2.iso | Vicidial 2.12-560a build: 160617-1427 | Asterisk 1.8.32.3