FaceTime is NO longer working on 1 ipad

Since a few days it’s NO longer possible to make a FaceTime connection with another iPad. Data connection is ok , on my iPad FaceTime rings but on the other iPad nothing happens, NO ringtone and NO FaceTime screen shows up. From the other iPad it is also not possible to initiate a FaceTime Call. iCloud works fine on the iPad so mobile data Is ok. What is the problem?