Situation: You've had a Galaxy S9 or S9+ for a long time with no issues. You have not installed any new apps recently. No app updates have occurred.
However, after a Samsung firmware update, the phone will (very frustratingly) no longer make voice calls. It will attempt to dial and give up after 10-30 seconds without ringing through.
Booting to safe mode does not help, but does remove all your widgets. (ARGH!)
Possible solution: The firmware update has enabled voice-over-LTE (VoLTE) calling support, but your carrier / access point doesn't support/allow for it.
To fix, go to Connections -> Mobile networks -> VoLTE calls and disable VoLTE calling.
You should be especially suspicious of this feature if you are using a femtocell or cell phone boost device. These devices probably don't support the newer VoLTE feature.
Oh, and hey to Google - F-you for removing all my widgets in Safe Mode. My God, you couldn't have saved their previous state somewhere to restore afterwards? Seriously, that is SO HARD?