We have 3 phones - my wife and I have Galaxy S10 phones and our daughter has iPhone 8. All part of the same Verizon family plan. When I call my daughter's phone it rings. When my wife calls her phone it goes straight to voicemail every time. Others can call my daughter just fine.
She does not have the "Do not disturb" option enabled.
The "Allows calls from" is set to "Everyone". Same setting on her synced iPad. Both do not show the call from my wife coming in. Can't figure out why.