“Number” in contact card is not available, for one contact only

Hi, I always use FaceTime to contact my sons. Suddenly, a couple f weeks ago, I saw the following message when I tried to call one of them (see photo attached). He is still able to call me using FaceTime.

I have tried deleting him from my contacts and starting over again, turned ipad and phone off then on again. Neither had any effect. Went into settings and turned FaceTime off, ipad off, ipad back on, FaceTime back on. Nothing!

Finally I completely deleted, then reinstalled FaceTime app, turning off ipad between these operations. No change. When I reinstalled the app, all the existing contacts and call history were still there. I definitely deleted it, not just offloaded it.

This problem is only with one of my sons and no-one else.

I have seen, on this forum, that other people were having this same problem as long as three years ago.

Please can you help? This is a real problem for me.

Many thanks