Can I limit a phone to only be capable of making extension to extension calls?

Access the Star2Star Web Portal   and navigate to CONFIGURATION (admin access required). Click Extensions and select the extension required. On the right side of the Extensions setup screen note the series of check boxes that provide outbound dialing access. Below left shows an example of the default extension screen. The example to the right shows the extension set with restricted access for extension to extension calls only.

(Note) Remember to click Save Changes followed by Save Changes to System when you have finished making changes

For full details review the Star2Star Advanced System Configuration Guide (page 11 - 12)