You can activate Call waiting in your Telavox mobile app through Settings > Incoming calls > When busy > Call waiting. Also remember to activate call waiting in the phone's own settings. You should therefore have calls waiting active both via Telavox and in the mobile phone itself. The same applies if you want the function disabled, then it should be disabled on both.

When call waiting is activated, you can receive a call even if you are already in another one. If you have problems, you can try restarting your mobile phone.