FaceTime says failed when I answer a call or when the iPad is answered it says user not available

My husband has been gone for a couple weeks with the iPad. We've been FTing for 2 weeks and then its stopped working. I can hear it ring but when I answer it, it says FT failed. Or if I call him, and he answers it says user unavailable. Now sometimes I hear it ring but there is now way to anwer the call, no window pops up or anything.