How to FaceTime multiple devices using same Apple ID

I have an old iMac I’m trying to FaceTime using the same Apple ID from my iPad. Both devices do NOT work with networks, so there’s no “phone” to dial. My grandmother has my iMac and I have my iPad. Both devices have my Apple ID signed in. She’s across a different state and absolutely would not know how to “sign in with her own Apple ID”. The best I can hope for is that she knows how to click the green button to answer my FaceTime call. Why can’t I FaceTime my own Apple ID on the iMac?