FaceTime greyed out for ONE contact

This is happening on the iPhone only.

For ONE contact, I can't access the FaceTime call button.

Also I cannot access that with the FaceTime app (top line).

I have been reading on this forum and others, and tried many options but nothing works.

That person changed phone number. Everything works on his side and with anyone else.

And same with me, except on my side, on the iPhone.

Because on the Mac, I can FaceTime with him and all the options aren't greyed.

Help, please.