Allow calls to be ended by the volunteers helping with the tablets

The volunteers (team tablet) will need to have two buttons that they can press to end a call, one that ends the call simply due to time (i.e. the scheduled slot has expired) and the other due to emergencies (e.g. patient is having issues and hospital staff members are now working).

The exact phrasing of the second button will be changed later, for now simply having this button is enough.