When calls are either missed or ignored, there is no prompt for the caller to leave a voicemail. Just dead silence.
One phone is an Xfinity Mobile LG Power, and the other a Moto G4. I am pretty sure both started the VM issue around the same time.
I have two other cell phones on the account (a Moto G6 and an Xfinity Mobile LG Charge) where the voice mail works properly.