Implement Call Waiting

Getting a call while on a call is currently very confusing. The phone rings loud, the answer/ignore popup appears but it is impossible to answer the call while being on another call. It is also confusing for the other person on the call because they can hear my phone ringing loud.

Therefore, we should implement the Call Waiting feature.

Waiting for that to happen, we may disable the possibility of getting a call while being on a call and just be notified that someone tried to call?