Please read How do I install a Softphone? if you have never installed your Softphone previously.
Also note that Softphones register with PBXtra differently than other IP Phones. Softphone registration requires the following information:
Display name: EXTN
Username: SOFTPHONEXXX
Password: EXTN
Authorization Username: SOFTPHONEXXX
Domain: sNNNN.pbxtra.fonality.com
[x] Register with domain
[x] Use as outbound proxy
[_] Manual override
[_] (SIP listen port) Manual override
When you have your Softphone open, please do the following:
Right-click anywhere on the GUI and select SIP Accounts
From the new window, select the first SIP Account listed
Click Edit
Review the settings displayed. Make sure they match the template above
Save your settings. Your Softphone should try to automatically re-register with PBXtra