iPad rings when iPhone is away from home

When out and about carrying my iPhone, incoming calls also are ringing my iPad, which is at home on my local WiFi network.

I thought Allow Calls on Other Devices was only supposed to work when devices were on the same network.

When I am at home, I want to be able to answer calls on my iPad and / or iPhone. But I don't want my WiFi iPad accepting calls when me and my iPhone are not there. (I didn't think it was even possible for this to happen).

Anybody have an idea what may be going on?