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It sounds like neither you, nor the person you call can hear one another.
I would start by resetting the network settings on the phone.
Reset network settings by tapping Settings > General > Reset > Reset Network Settings.
- Note: This will reset all network settings including: previously connected Wi-Fi networks and passwords, recently used Bluetooth accessories, VPN and APN settings.
From: iOS: Troubleshooting Wi-Fi networks and connections
If that does not resolve the issue, I would perform steps 1, 3, and 5 of the iPhone Troubleshooting Assistant found here http://www.apple.com/support/iphone/assistant/phone/#section_1.
If you get to step 3, I would close all the apps that are running. Here is step 1 to get you started:
To restart iPhone, first turn iPhone off by pressing and holding the Sleep/Wake button until a red slider appears. Slide your finger across the slider and iPhone will turn off after a few moments.
Next, turn iPhone on by pressing and holding the Sleep/Wake button until the Apple logo appears.
Is iPhone not responding? To reset iPhone, press and hold the Sleep/Wake button and the Home button at the same time for at least 10 seconds, until the Apple logo appears.
If your device does not turn on or displays a red battery icon, try recharging next.