Step 1: Create a radius server - https://help.ui.com/hc/en-us/articles/360015268353-Configuring-a-RADIUS-Server-in-UniFi

Step 2: Create a VLAN for the phone network - https://help.ui.com/hc/en-us/articles/9761080275607-Creating-Virtual-Networks-VLANs. This will contain all of your phones and your Hello Hub or Cloud Connector. Additional required for WiFi phones to connect. VLAN will use the MAC addresses for your devices to associate them to the VLAN.

Advanced Settings for Dream Machine Pro (UDM Pro)

Search for SIP or Timeout. Adjust as below.

Under Firewall Connection Tracking (bottom of below image), make sure that SIP is not enabled. If there is a gray box showing SIP, that means it is enabled and you must click the X to dismiss/disable it.

Timeout should be set to a number greater that 600 seconds. 620 is recommended as our phones use 600.