Download in install the application here:

Go to the Endpoints Screen for the extension you wish to program.

Be sure that you have an endpoint that is of Type=IP.  The SEP Number and Password will be displayed.

Launch 3CX and in the Account settings section.

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: