Calls from Favorites go to voicemail after one ring

My daughter has an iPhone 11. Recently calls from her Favorites ring once and then go to Voice mail. We have tried toggling stuff like Silence Unknown Callers on and off, but we are not Settings experts. And Google offers up a lot of bad advice from Apple Discussions. Any help will be appreciated.

It’s most likely a carrier capacity issue and the “Favorites” setting is a red herring.,

Do a test by taking a couple of favorites off favorites and see if it rings more than once.
Then make them favorites again and test again to check number of rings.