How to stop getting sound of another phone call
You can disable "knocking" at all – so that the second caller gets a "busy" signal, and you a "missed call" notification. If that's what you want, depending on your device the setting might be in the phone app:


open the phone app
tap the overflow icon (the 3 stapled dots in the upper right corner)
select Settings
chose Advanced Settings


There you should find the option. If it's not there, an USSD code can toggle that:


open the phone app
pretend you want to make a call to the number #43# (enter that code and tap the call button)


Now it should be turned off. If you want to turn it on again, use *43#, to check the status *#43#.