Essentially, the user joined the Carrier Services beta through the Play Store. Once you select “Join,” the app will inform you that it’s enrolling, as it does with other beta programs.
The Reddit user also notes that they took additional steps to ensure the method would work. They forced stopped the app in app settings and cleared the app data. You can do this by navigating to Settings > Apps > Carrier Services. You should see the option to force stop the app while clearing the app data requires navigating to the app’s Storage & cache setting.
You may need to reboot your phone before you notice any changes, but many users on the threat have noted that the fix has worked for them. One even posted the Carrier Services beta APK link for those that would prefer a direct download (install at your own risk).
It’s not a surefire way to fix the connection issues, as some have indicated that it did not appear to work for them. However, in lieu of an official fix from Google, this may be the only option for those having trouble making calls after installing the December update.
That said, Google has acknowledged the connection issues and is actively investigating, so it may not be long before an official fix is pushed out.