No experience whatsover for configuring these devices.
https://help.ui.com/hc/en-us/article...cation#add-sip

Have you checked out this YouTube configuration video for Unifi Talk? A different device, but it might help.
https://www.youtube.com/watch?v=fSbJ2FBF1MA

The SIP proxy server should be handling both incoming and outgoing as long as you are registered to the server. I noticed in the video he was using the IP address of the proxy server instead of the DNS name for most of the field entries. sip.voipwelcome.com = 209.212.145.160

See what happens when you use this IP address in the fields (proxy, ACL IP, etc.)

Good luck!