You might want to give the following a look to see if it perhaps has somehow gotten reset.
With the Connect App open, select the three horizontal lines in the upper left corner of the screen and then choose the Settings menu option. From the screen that opens up, select the Notification Preferences option. Down at the bottom of the screen you are taken to should be a Voice2Go heading with Incoming Calls beneath it. Make sure that option is set to On, which at least in my case means the colored dot is to the far right and is a very bright blue color.
Additionally, at the very top of the screen you might want to check the All Notifications setting and make sure it is set to On as with it off it automatically turns all notifications off.
At the actual Android level, select the Settings and then select Notifications. Scroll down the list of apps until you get to the Connect app and select it. Make sure the Block All setting is not turned on.
This continues to be an issue. The xfinity connect app was causing issues with my other apps where I wouldn't get notifications so I had to remove the app from my phone. I still have it on my Google Pixel tablet and will try again if the app has a new update. It also doesn't work properly on my Google Pixel table either.
In both cases once I sign in it works where I get notified that an incoming call is coming into my xfinity voice line. But after some length of time it seems that it gets disconnected from the xfinity service and no longers works. I can clear the apps data re-sign in and then it works for a little while, then nothing after some length of time.
I hope they fix it, but since it's been almost a half a year it seems they are no longer investing in this app, at least not the android side.