There are 4 radios in a typical device: cell, bluetooth, wifi, nfc.
This app allows you to just shut off the cell radio while leaving everything else on.
Airplane mode shuts off all radios and there is no built in method to disable just the cell radio.
Why to disable it?
1. When your cell radio can't access a tower it will drain your battery
2. Your location is tracked by carriers using your cell radio (This is necessary so people can call you.. it's not that they are evil)
This app has been tested and works on the HTC Dream. This does not work on the nexus 4 or the galaxy nexus.
If this doesn't work with your phone you can access a builtin interface which has the same option. If you dial *#*#4636#*#* in Phone it will pull up a menu that will allow you to shut off the cell radio.
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