When you are in a call you can choose what happens when a second person calls you. You can choose a different setting for one of your numbers or have the same setting on both of your numbers.
For example, when you are in a call, a message can be played when they call you on your mobile number. If they call you on your fixed number, the call comes through
Click on your profile picture and gear-icon -> Telephony and Call waiting & busy handling Desktop View 
Mobile View
To activate Call waiting click on the switch so it becomes green
To inactivate Call waiting click on the switch so it becomes blue
Choose what's going to happen when you are in a call
- Busy tone
- Play a voicemessage and hang up
- Go to voicemail*
- Redirect a number*
*You can choose to play a voice message before the call goes to your voicemail Desktop View 
Mobile View
