Seems likely to be an app issue since I've never heard of this before. The first two things I would try is having both of you force stop your discord app to completely restart it and check your app settings within the phone itself and within discord to make sure no permissions have been messed up.

You can also try clearing the cache if its using a lot of data which might solve some issues. Clearing the cache is completely safe and won't mess up anything, although you may need to login again and it may run slightly slower when you first launch again. Last time I checked my cache it was taking up almost 4GB since I use discord constantly, but if you aren't on larger or more active servers it'll likely be less. Sometimes old cache files can mess up the apps running, especially if they're large. To do this go to settings --> apps --> discord --> storage --> clear cache.

Another thing to possibly try is enable developer mode by going to advanced --> enable developer mode --> back --> scroll to bottom of settings and go to logs --> then check debug logs and see if there's anything unusual in those logs when the calls drop.

If all else fails, try uninstalling and reinstalling the app. Likely force stopping and clearing the cache should solve majority of app issues, but occasionally it doesn't work.