Yes, I have a 6s and she has a 5s. It does not occur on every incoming call and on the calls it does occur it is with familiar and random callers
its probally only happening when your getting a call form another iphone thats why it doesnt happen all the time. tell her to make her own icloud and imessage account and see if that resolves hte issue. i bet it will
There is also a setting in the iMessages area if they are sharing the same Apple ID to ring to all devices associated with the same Apple ID.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.