Me and my partner are using an iPhone 6 plus on Optus, And we regularly use face time. Last week she's gone to hongkong and has a new sim and I have tried using FaceTime being unsuccessful, the face time button does not show regardless added her new number any ideas?
Check your Facetime setting. Setting -> Facetime. Facetime can be turned off on her side.
Also check the associate apple ID is still correct and the list of available contact/email that can be reached by Facetime. When switching sim card, these settings are normally affected.
Obviously, make sure her sim card has 3G/4G data capability.