In UK the following will apply.
Setting 7500 will result in a discrete enquiry from ATC “(Callsign) confirm you are squawking assigned code (code assigned by ATC that it would otherwise be squawking).”
Setting 7600 will merely indicate a comms loss with the expectation that you will will follow any published comms loss procedure.
Setting 7700 will require verbal explanation of the nature of the emergency and any resultant requests/intents.