Wifi calling on old iPhone

I purchased a new iphone 6s and setup wifi calling (with At&t). it works well with my laptop and ipad. I still have my old iphone 5s and was wondering if i could use it as another device for wifi calling. on the phone settings page on the 6s it shows up as one of the "Allow calls on other devices". The problem is when i go to the 5s and go to facetime, it doesn't show me the calls from iPhone option. All devices are on IOS 9.2, so that shouldn't be a problem. is this by design, or am i missing something?