I just went through all the settings on my iPhone. The only one that "possibly" could be causing the problem would be (on your iPhone)
Settings, FaceTime (on), and iPhone Cellular Calls (on)
That would allow FaceTime calls to made from your Mac when it's nearby on the same network and signed into your iCloud account. Those FaceTime calls could be made directly from your contacts list on your Mac.
If it's not that, I really have no idea what could be causing it. Maybe someone else can chime in here and suggest something.