Divert to voicemail means that.
The VoIP access boxes relate to the fact that VoIP services are portable. For example I sometimes use the service from abroad, set up on a laptop or tablet. This means that anyone who has managed to steal your credentials can put a substantial bill onto your account, usually from an IP address from an another country. The restrictions are to hinder this fraud. If you do travel, you can relax the restriction while you are away. Incoming calls are not affected.
It's also a good reason to restrict external access in your router.