No call waiting/feedback the person I'm calling is on a call

matrix-react-sdk version:
riot-web version: 1.0.4
olm version: 3.0.0

So I was on a voice call with a friend, both of us desktop versions of Riot.

Riot on my computer crashed, but his client was still in the call.

I tried calling them back, and the Riot app kept ringing and ringing on my end to infinity, and on my friend's end on the desktop no notification was sent to the user, but his phone was receiving alerts of an incoming phone call.

So, there's some shortcomings here:
1. A single account can be on multiple phonecalls on multiple separate devices, this could lead to phone routing mishaps
2. We need a way to do call waiting, to pick up when someone calls you while you're on a call
3. We need a way to tell if the person we're calling is already on a call
4. I could really use a block of Mozzarella cheese if that'd be feasible

So, hopefully this helps! And with any luck I might find some cheese along the way! Yay! XD

Reproduce the reported callback scenario after one desktop client crashes while another device remains in the call, including the desktop and phone notification behavior. Done means the issue's requested call waiting and in-call feedback behavior is defined and works consistently across the affected clients.

Written by the indexing model from the issue text.