Android
Turn Call Waiting On or Off auf Android
Call waiting sits inside your carrier's network settings, but Android gives you a switch to turn the feature on or off from the phone app. When it is enabled, an incoming second call plays a short tone and shows a pop-up while you are already talking to someone. When it is disabled, the second caller usually hears a busy signal or gets sent straight to voicemail.
Your carrier decides the default state, so the toggle may already be on when you first check your phone. Changing it yourself does not affect your existing call, only how future incoming calls behave while you are on the line.
To reach the setting, follow this path:
Tap on Phone ➔ Open the Menu ➔ Tap on Settings ➔ Tap on Calling accounts ➔ Choose a SIM card (Dual SIM) ➔ Tap on Additional settings ➔ Enable or disable Call waiting
On dual SIM devices, repeat the steps for the second SIM if you want the same behavior on both lines. If the toggle is grayed out or the second call still does not come through after enabling it, the issue usually sits with the carrier's network configuration rather than the phone settings, and contacting support is the faster fix.