You can however add multiple extensions to the mobile app. In iOS (at least the not-the-new app) tap your name in the upper left to quickly change to another extension, or use menu->Settings->Accounts. Just have to remember to change back to "your" extension afterwards.
Or, dial the voice mail number (999), press # to log in as the other user, and 9 to get to the menu to change status.
Founded in 2005, when VoIP was an emerging technology, 3CX has gone on to establish itself as a global leader in business communications.