When I first got my Galaxy S8 for Xfinity Mobile, the voicemail app that it came with seemed to work just fine, and I was able to listen to, and read a transcribed copy of, messages that arrived. Then around the beginning of the month, when starting the app I could briefly see my list of messages, then that would be replaced by a screen reading, "Oops! Uh-oh! Looks like there was some trouble with your account. Not to worry. Please contact Support and we'll get this cleared up right away!" and provides a "Call Support" button. When this button is tapped, a call is placed to 215-845-5845, which goes to a recording saying it is a non-working number.
A call to Xfinity Mobile help elicited no solutions. I have since installed the Youmail app and am using that instead, identifiying the underlying phone network as Verizon (which it is). That seems to work well for me now. I am posting this here as a suggestion, for others struggling with the Xfinity Mobile Voicemail app.
I've never had problems with the XM Voicemail app on the LG V20 US996. I just left a voicemail from another phone to check after reading your post. It works well for me. I'm running XM Voicemail app version VVM-R1.6.3. I suggest you remove (uninstall) the update then reinstall it.
I tried reverting the update that was applied to the Xfinity Mobile Voicemail app, and now the app simply shows a screen with the words "Please download the XFINITY Mobile Voicemail application for them Google Play Store". But when I go the to Play store, it says the app is already installed. In other words, Xfinity Mobile Voicemail seems like a dead-end at the moment.
Ah, well, I didn't need to, the app was automatically updated after a restart, and now the Xfinity Mobile Voicemail app again behaves exactly as described in my initial post. So it appears there is no fix for the bad behavior as yet.