Something like this happened to me recently. It drove me crazy! riI found out I was signed in on Icloud on both devices under the same icloud account and somehow that caused the ipad to ring whenever my phone rang. It also sounded whenever my phone got a message. That could be the problem.
You need to go into settings and adjust your FaceTime options on your iPad. Although it says FaceTime it will cause your iPad to ring if you have the same contact options on both devices. This also happens when you share the same Apple ID on both devices. You could opt to create a different Apple ID for the iPad and then just use family share so you can share purchases on both devices...or you can go in to settings like I suggested and adjust things.