The red "hook on" key is used to end phone calls, and if you are not on a phone call but in some application it switches context from that application to the telephony application, which is the main screen on your phone.
In the emulator I can catch presses of this key with
TKeyResponse CDialGenieDoubleList::OfferKeyEventL(const TKeyEvent& aKeyEvent,TEventCode aType)
{
if (aType == EEventKeyDown && aKeyEvent.iScanCode == EStdKeyNo) // Red, hook on
{
iParentApp->HandleCommandL( EDialGenieCmdBack );
return EKeyWasConsumed;
But running on the the target (Nokia 3650) I lose context without OfferKeyEventL being called.