With my iPhone turned off, I tried calls from the Ultra using APP2. They sound just as good as if coming from the iPhone. On speaker, it's more than decent for short calls. I did notice the Ultra loses connection where my iPhone doesn't. Probably cause no 5G.
Yeah the phone calls on the Ultra are remarkably good, the speaker on it is excellent for such a small device. I also use the walkie talkie function with my family a lot.