Back in the day I found a guy who cloned the identity of my analogue portable cell phone with my in-car cell phone so they both behaved as the same number. When one rang, they both rang.

In theory when I answered a call on one, the network would be signalled to say I'd picked up and it would stop telling the other to ring - but once in a while the other phone would keep on ringing .

iirc phone cloning may have been illegal here in the UK at the time...