I've seen several posts on Reddit and other forums that say "oh you can use a grandstream phone with UniFi Talk and it's flawless". Unfortunately, I am not a VoIP engineer so it was not intuitive to me, but I got it to work. Here's how.
For this writeup, I'm using a Grandstream DP750 DECT base with a DP720 DECT wireless handset. I really wanted a wireless phone for general use in my house, but Ubiquiti doesn't make one, so I wanted to find a way. My goal is to setup UniFi Talk for my home phone solution and share a single phone number with a UniFi wired phone that sits on my office desk. As far as I know, you have to get a UniFi phone to do the initial Talk setup (though, in my tinkering, I noticed it's using a PostgreSQL database, you could probably bypass the initial setup if you knew what you were doing). I'm also using a UDM-Pro for my gateway.
Assuming your UDM-Pro management interface is on 192.168.1.1, your base station should be assigned an IP on a VL