I'm assuming you're referring to the RTH button on the front of the remote controller. If so, pressing and holding that button will initiate the Smart RTH function -- which will return your Phantom to the last marked home point as long as your remote controller is connected to your Phantom and your Phantom has a good GPS signal.
The Phantom manual does not mention what will happen if you initiate Smart RTH and your Phantom does not have a good GPS signal. I'm assuming it will either not initiate RTH or your Phantom will obey the following DJI GO setting:
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When Fail Safe RTH is activated and your Phantom does not have a good GPS signal, it'll land at its current location after 10 seconds when "Return-to-Home" is selected in the above setting.