iPhone 16 sends land line calls directly to voicemail but mobile numbers ring through

Check your settings at Settings > Apps > Phone:

• Screen Unknown Callers
• Call Filtering

Since a phone number calling you is a phone number whether from a landline or cellphone, your phone cannot know which is which. But you could have Silence Unknown Callers turned on? You could be using Call Screening Features, if you are updated to iOS 26.

Settings > Apps > Phone > Screen Unkown Callers and Call Filtering - choose the options which best meets your needs.