If your iPhone ends an active call when another call comes in, it's likely due to the Call Waiting feature being disabled.
To resolve this, navigate to Settings > Phone > Call Waiting and ensure the toggle is turned on.
This setting allows you to receive incoming calls while you're already on a call, preventing the current call from dropping when a new one arrives.
If the option isn't available, it might be a limitation imposed by your carrier, in which case contacting them for further assistance is recommended.
Additionally, ensure your iOS is up to date by going to Settings > General > Software Update, as software updates often include fixes for such issues.
If the problem persists, consider resetting your network settings via Settings > General > Transfer or Reset iPhone > Reset > Reset Network Settings, keeping in mind that this will erase saved Wi-Fi passwords and VPN settings.
Should none of these steps resolve the issue, reaching out to Apple Support or your carrier may be necessary for further troubleshooting.