[Freeswitch-users] detect registered user on ESL

I'd using incoming ESL connection, and would like to know when some user using a sip application from his cell is or isn't connected. I thought about listening for sofia::register/unregister events to decide if he's online, but then how can I tell if his phone doesn't answer pings/shutdown? maybe I should use sofia_presence_data? or somehow incorporate PRESENCE_IN event? Thanks, Shlomi Agiv