After pairing, your cellphone's audio is routed to Bluetooth (Cell2Jack/Home Phone) by default.

If you like to use your cellphone to talk, just select your cellphone's speaker from your cellphone's screen
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For iPhone, You can automatically re-direct the audio of phone to the iPhone speaker. iPhone can also automatically answer calls after a length of time you set. Go to Settings > Accessibility > Touch > Call Audio Routing, then choose an audio destination.

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