Log in to your IP Phone. Follow the instructions here:
Sample configuration of SPA921
Change and update the following settings that can be found under the "EXT 1" Tab (ensure you have hit the "Admin Login" link)
General
Line ENABLE = yes
Proxy and Regstration
Proxy: End Point Server
Subscriber Information
User ID = SEP Number
Password = SEP Password
Note:
SEP Number and Password can be found in your Slingshot Dashboard under:
Team Members->Endpoints Page
Ensure that the Type is set to IP