It sounds obvious. In an emergency (in most of North America) dial 911.
Because of the differences in operating characteristics, transmission pipelines have different safety risks and concerns for emergency response, including the pipeline company’s ability to shutdown the pipeline rapidly.
In a manner similar to the Enbridge situation in Michigan last year (the NTSB report on that incident is still pending), the PG&E data system showed a pressure drop within four minutes of the rupture.
PG&E dispatched a single technician to the scene who was not authorized to shut off valves.
It was not until a technician arrived at the scene and reported in some 16 minutes after the event began that PG&E control room put together the drop in pressure, alarms and dispatcher information and realized that they had a major problem.
A pipeline operator’s prompt notification to the local emergency response agencies through a 911 emergency call center can be crucial to the success of the emergency response effort and protection of the public. Even in the case of a smaller, slower leak that does not immediately ignite, when the pipeline operator has immediately notified local emergency response authorities of a potential serious problem, can mobilize needed response resources and area better able to recognize quickly the symptoms of a potential serious gas leak threat
Apparently under current US regulations, there is no requirement for pipeline operators to call 911.
The report goes on to say
The NTSB is concerned that a pipeline operator that does not require control room operators to notify the applicable 911 emergency call center in the event of a possible pipeline rupture can adversely affect the timeliness and effectiveness of emergency response effort. Therefore, the NTSB recommends that the Pipeline and Hazardous Materials Safety Administration issue guidance to operators of natural gas transmission and distribution pipelines and hazardous liquid pipelines regarding the importance of control room operators immediately and directly notifying the 911 emergency call center(s) for the communities and jurisdictions which those pipelines are located when a possible rupture of any pipeline is indicated.