Hi James5522,
You started from a good resource: If you can't make a FaceTime call on your iPhone, iPad, or iPod touch - Apple Support
As the issue only occurs with certain contacts, test signing out of FaceTime on your device and restart your iPhone. Sign back in with the steps below found in the article here: Use FaceTime with your iPhone, iPad, or iPod touch - Apple Support
"Turn on FaceTime
Open the FaceTime app and sign in with your Apple ID. You can also do this from Settings > FaceTime.
If you're using an iPhone, FaceTime automatically registers your phone number.
If you're using an iPhone, iPad, or iPod touch, you can register your email address:
- Open Settings.
- Tap FaceTime, then tap Use your Apple ID for FaceTime.
- Sign in with your Apple ID."
You can also view the Apple Recommended post here as they recommend removing and adding back the contact card as well: I keep trying to facetime someone and I g… - Apple Community
Cheers.