Problem
Users cannot see when another call is coming in if they're already on a call.
Solution
Edit the settings for the Contact Centers in Dialpad.
Change contact center settings
Go to your Dialpad account.
Click Admin Settings > Contact Centers.
Under Agent and Call Settings, uncheck all three boxes for the routing. The changes are saved automatically.

Now when a call comes in, agents will be notified of any inbound calls, even if they're on a call.