If you lose your phone you should let your monitoring team know as they may think they can still contact you with it, or think that the location of the phone represents your actual location.
In many cases, your monitoring team will want to deactivate your Smart24 app to avoid any confusion. They can do this from the Grid Administration Module.
In some cases, for instance if the Tracking feature was on, your monitoring team may be able to assist you by telling you the last known location of your phone.
iPhones also have a feature called 'Find My iPhone' and 'Send Last Location' that can be turned on from Settings, that may be able to help you recover a lost phone.