Although Perksie may disagree
I think I am right in saying that was a code used on some older phones from certain manufacturers (e.g. Nokia) to change the codec the phone uses, in effect changing the quality of the call.
I think 3370 increased the call quality by using an enhanced codec and 4720 decreased the call quality by using a lower codec. I can't remember whether it was *3370# or *#3370 or whatever.
The result of lowering the codec was that phone would use slightly less power effectively draining the battery more slowly.
There is no hidden or reserve battery that is just a misunderstood urban email myth just like the car key question someone has asked here...
viewtopic.php?f=2&t=53083&p=316836#p316836The are many codes that are undocumented.