To use the iPhone speaker phone option, simply hit “Speaker” on your phone. If that doesn’t work then follow these steps:
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Make sure nothing is plugged in to the headset jack such as adapters or headphones.
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Check to see that the Ring/Silent switched is not set to silent.
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Go to the Home screen and make sure the volume buttons are turned up. While doing this, if you see an icon showing (headphones) that means your iPhone thinks something is plugged into your headset jack. Double check to make sure no debris or other object is lodged in the jack.
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If you find there is hard to remove debris that makes the phone think there is something in the headset jack you might have to have it serviced. You could connect and then remove a pair of headphones a couple of times to see if that removes the debris before having to take it in for service. You can also check to see if any protective film on the display is not covering the receiver.
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The receiver can be cleaned with a clean, soft bristled brush such as a soft toothbrush to brush debris away.
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You can also check to make sure that Bluetooth is turned off by choosing Settings > General > Bluetooth
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From this point, restart the iPhone and if needed, restore the device. If nothing else works, go to the Service Answer Center for further service.
Source: Apple Support