How to put a call on hold and answer another call on Android and iPhone

The ability to put a call on hold and answer another call is built into the default phone apps on Android phones and iPhones.
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Use O+ Connect to Effortlessly Transfer Files Between Android and iPhone, Windows & Mac

O+ Connect allows you to transfer files from Oppo, OnePlus, and Realme devices to iPhones, Mac, and Windows for one-click transfer in real time.