In my company we always have agents or employee's on site you never know when an aircraft may have to return to stand due tech problem, perhaps even disruptive passenger.
Doesn't mean at 1am when rain is peeing down in a a major storm at Gatwick they will answer the phone.....does it ?
What then ?
Do you abandon aircraft and go seeking them ? or stay with the aircraft and keep ringing.
Me at a guess you would keep trying as I figure the FR crew were doing but when no access to terminal, handling agents won't pick up the phone and limited options what then ?