You are going to need the extension you are trying to register to your phone, the domain provided through the e-mails and the secret of the selected extension.
Click tools, then options.
Click account and then click Add.
Select Freevoice for the network.
Input yourextension@yourdomain in the User ID field.
Input the secret of the extension into the Password field.
Your extension should now be registered.
You can also check if a users softphone is registered to its extension without accessing that users workstation by logging into your Admin account on your PBX, then going to:
PBX > PBX Info, then click the SIP Peers tab.
You should see information similar to the image below, which tells you: