Q: Iphone 5 and 6 problem

This is a feature of continuity/handoff and happens when you are using the same Apple ID across multiple devices. You can:

1. Disable handoff:

Use Continuity to connect your iPhone, iPad, iPod touch, and Mac - Apple Support

OR

2. Do not share the same Apple ID. If you want to share content across multiple Apple IDs use the family share feature:

Set up Family Sharing - Apple Support

It's not "Handoff", it's "Phone Calls".  See the continuity article article FelipeV posted, but look for Phone Calls section, specifically:

Turn off iPhone cellular calls

To turn off iPhone cellular calls on your iPad or iPod touch, go to Settings > FaceTime and turn off iPhone Cellular Calls.

On your Mac, open the FaceTime app and go to FaceTime > Preferences. Click Settings and deselect the iPhone Cellular Calls option.

Alexa,

Apple strongly recommends that everyone use their own Apple ID and not share one. It is like your ID card at work or your Social Security number, not meant to be used by two people.  However as a quick “fix” go to Settings, FaceTime, and turn iPhone cellular calls to OFF.

Best,

elcpu

Alexa, at this point my recommendation is for one of them to get a different Apple ID, this is easy to do and unless they have made purchases from the iTunes or App Store, the different ID will not impact much.