Originally Posted by
MechEngr
The only downside I foresee is battery life. When the phone is shielded or unable to connect to a tower they mostly compensate by increasing the signal output power, draining the battery at a pretty good clip. If there was an app that would control that aspect it is certain that the phone has all the components to do even more than a tag does; tags depend on being discovered by cell phones and it is the cell phone that provides the location data. Using a phone instead of a tag skips that intermediate step.
Powerbank?
I have an old phone that normally can go better than two days if left alone, I guess it depends on how long you need it to last. The only problem with that phone is deteriorating volume is why I changed it so getting it to make a noise remotely would not be very good.