You do have a case. You must present evidence to the court that the reason that you violated the 2-occupant minimum in the carpool lane was that there was a life-and-death necessity that required that you immediately get to the hospital where you work. If you bring some corroboration in the form of a signed declaration from the individual who paged you, you will increase your chances of winning. It would not be a bad idea to hire an attorney who does traffic tickets to handle this case for you. Remember that a carpool violation is not a moving violation so you will not be required to attend traffic school and you will not get a point on your record if you are found by the court to have violated the carpool law. If you crossed over the double yellow line which separates the carpool lane from the other lanes, then that is a moving violation and you will either get a point on your record or you will need to go to traffic school.