Hi there, jjboulder!
Thanks for using Apple Support Communities for this issue. I see you're trying to use the new Wi-Fi Calling feature on your iPhone 6s after upgrading to iOS 10. I see that the connection drops after a bit but I'll be glad to help with this.
Make a call with Wi-Fi Calling
If you can't turn on or use Wi-Fi Calling, make sure that your carrier offers Wi-Fi Calling and that your devices have the latest software. Then, follow these steps if you can't turn on Wi-Fi Calling or make a Wi-Fi call. Wait two minutes before you go from one step to the next.
- Go to Settings > Phone > Wi-Fi Calling and make sure that Wi-Fi Calling is on.
- Restart your iPhone.
- Connect to a different Wi-Fi network. Not all Wi-Fi networks work with Wi-Fi Calling.
- Turn Wi-Fi Calling off and then on again.
- Go to Settings > General > Reset and tap Reset Network Settings.
FaceTime isn't available in all countries or regions.
Cheers!