Hi
@tlm16 , you're absolutely right. I've had this feature in the past, when i was using Samsung S2 and S4. I'm using Xiaomi now and this feature is nowhere to be found. Not in the phone nor the sim settings. I've even checked with my service provider, they don't seem to understand.
@jayeff , I can vouch that I've used this feature before mate. It's a small notification on the screen, stating “call is waiting“, when i dial a person, who's already engaged in another call.
Regards Grid