Devices must be on same wifi network to FaceTime

My mom has an iPad Mini (it is 3g enabled but is not connected to a data plan) and when she tries to FaceTime me she gets a message that pops up saying "The devices need to be on the same wifi network". We used to be able to FaceTime each other from across the sate with no problem (her with her iPad and me with my iPhone or mac) but for the last couple months this message has been popping up every time she tries to call with FaceTime. I believe she has iOS 8 installed on her iPad but I'm not sure which version. How do we fix this so we can FaceTime without being in the same house connected to the same wifi network?