Can my contacts know if I'm on another call when they try to call me Will they know if I'm at another call even if it's not using the whats up app? On a regular cell call it would just keep ringing if the person is on another call. If you have call waiting you know they're calling you. I don't know if I explained myself.
It depends on your service provider.

In my country, all the networks will inform the caller that you are on another call via automated response; they will be asked to either hold on or call you back.

Hope this answers your question.