Option to disable ringing while in call?

Greetings!

We have the following situation: One of our company employees gets called, accepts the call and talks with the customer. Another customer calls, while the employee still talks with the first one. The problem is, the second call still plays the notification sound while our employee is talking.
Is there a way to disable notification sounds during calls? Other (paid) softphones have a “Ring when talking”-feature that can be disabled. I couldn’t find an equivalent in Jitsi.

Thanks!

net.java.sip.​communicator.impl.protocol.SingleCallInProgressPolicy.enabled=true

That doesn’t seem to work for me. I added the value to the property editor (it didn’t exist yet) and restarted Jitsi. Do I have to do anything else after adding the value?