Q: Can't Delete iCloud eMail from iMessages on iPhone iOS 9

I have an iPhone 5 that I logged into with my iCloud account. I am trying to make it that this phone only uses my phone number for iMessage and FaceTime as my iPad uses the eMail address and I would like to keep the two devices seperate. When I go into Settings>Messages>Send&Recieve, I click on the 'i' button next to my iCloud eMail address and click remove this account. My iPhone then says "this email address could not be deleted". Why? Why is it that I can't use my iCloud eMail address for my iPads Face Time and iMessage, and use my phone number for my iPhones iMessage and Face Time.

HELP PLEASE!