If I were you I'd write Total describing the incident then, ask them to:

a) take off all the signs because their employees are not taking them
seriously, or
b) take them off because it's a myth
c) ensure that all employees conform to the policy

Personally, although it was shown in Mythbusters that a reliable cellphone won't cause much, we've also read about defective or substandard batteries that could explode, e.g. the recent Sony recalls, or some fake Nokia replacement batteries. So I'd go with c) as we will never be sure that everyone's cell phone's batteries are in good condition.