If you wish, you can bar outbound calls on an extension. This will make the extension 'internal only', i.e. it can only call other extensions on your account. You can, however, still route inbound calls to it.
This functionality is useful if you have an extension that doesn't have a need to make outbound calls (for example, a user in a call centre answering inbound queries only).
After logging in to your control panel, select Extensions.
A list of extensions on your account will load. Select the one you wish to disable outbound calls on by clicking the seven digit internal number alongside it.
The extension configuration page will load. Select PSTN/Billing Settings to expand the sub-menu. At the top is a tickbox called PSTN Access. Untick this box and then click Save to only allow inbound calls to the extension.
Calls to the emergency services
As a safety measure, if you have disabled calls to external numbers on an extension then calls to the emergency services will still connect.