The VoIP phone "registers" with our servers on a frequent basis.  This registration lets our server know where it should send inbound calls. 

You can tell if your phone is registered. If it is not registered, follow the Network Troubleshooting steps

Yealink Phones

registeredyealink

On Yealink phones, the phone icon is filled wither gren or grey when registered, and ic unfilled when not registered

Grandstream Phones

registeredgrandstream