Bad Connection on iPhone 6 to iPhone 6s or iPad, Working fine with iPhone 5

Hi,

when i'm using my iPhone 6 or iPad mini to call a family member using iPhone 6s or iPad, Facetime always has Bad Connection. Calling a iPhone 5 instead (Same Network As iPhone 6s) It works without Any Problem. Any ideas?

ios is up to Date on every device, Internet Connection works without any Problems....