After pairing, your cellphone's audio is directed to Bluetooth (Cell2Jack/Home Phone) by default. 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.
For Android's user, if you can't find the setting, just select Bluetooth (Cell2Jack) as audio device from your cellphone's screen after the call connected.