You can suppress the call waiting tone for external calls. 

Deactivating the call waiting tone

Press the Talk key. 

Call the system menu. 

Select and confirm the menu item. 

Press the On-hook key to end the process. 

Activating the call waiting tone

Press the Talk key. 

Call the system menu. 

Select and confirm the menu item. 

Press the On-hook key to end the process.