I just setup Xlite with my credentials and it works as we expect it to work. With incoming calls, when I pick up the any of my phones or softphone, the other phones stop ringing. Audio is perfect too on the softphone.

Here is what I used.
Display Name
UserName
Password
Domain - east01.voipwelcome.com (which is where my ATA is registered)
Checked "Register with domain and receive incoming calls"

In Topology section I set "Manually specify range" to 6060-6061. Doesnt really matter which port you set to. My PAP2T ports are 5061 and 5062. Just didn't want to clash with those ports.

In Advanced section, I set it to re-register every 60 seconds. Send keep alives and User port buttons are checked.

When the softphone registers, you will see 3 entries in the vPanel->Devices page. When you exit the softphone, that extra entry will go away.