Download in install the application here:
http://www.3cx.com/voip/softphone/
Go to the Endpoints Screen for the extension you wish to program.
Launch 3CX and in the Account settings section.
Credentials
Extension: SEP Number
ID: SEP Number
Password: SEP Password
My Location
Select: "I am out of the office - external IP"
Enter: Proxy Server
Press OK. The phone should show "On Hook" and "Available". See below: