This article specifically relates to the Windows version of Gizmo5. There are similar articles for Linux and Mac.
To dial out on your Gizmo5 Phone you first have to understand whether you are dialing a Gizmo5 User or a Telephone Line.
Just enter the Gizmo5 name inside the dialing window.

You can also add this contact to your phonebook. For more information on how to add a contact please refer to the knowledgebase article. How to add a Contact?
To dial a telephone line. You will need to enter a phone number into the same Dialing window.
A United States number can be dialed in the following manner:
1-(XXX)-XXX-XXXX an example being
1-(858)-555-1212
If you need help dialing internationally please refer to our knowledgebase article. How to dial internationally?
To call to or from another SIP-based VoIP provider, you must dial a
SIP URI.