In this day of handheld computing power and the ubiquitous "smart phone", it is worth remembering that our "911" emergency response system was built around the old fashioned "landline", and as such calling 911 from your cell phone may not go as planned. Here are some tips to be mindful of, so emergent help can find you in a timely fashion if you need it.
Years ago, 911 calls from a mobile phone were actually routed to the California Highway Patrol (CHP), to a dispatch center often several counties away. This has changed as the technology has improved. Now, if you have your GPS location enabled, your location should be calculated and your call should be routed to the appropriate local dispatch center. However as a back up, at least in San Mateo county, you will specifically be asked for your cell phone number and location to confirm both.
In order to bypass this still slightly uncertain step, you might add "911 Emergency" as one of your "Favorites" (Apple iPhone terminology) and enter the phone number for your local emergency dispatch center so you can dial them directly. For San Mateo County, that number is (650) 363-4915. For Santa Clara County, (408) 299-2501.
Of course the 911 system should be used responsibly, but my advice would be, if in doubt, call 911! Once you've made the decision to call for help, if safe and convenient, choose a landline over your cell phone to help facilitate location finding and communication. If you use your cell phone, speak calmly, and be clear about why your calling, where you're calling from, and your call back number. And stay on the line!
Lastly, consider dialing emergency dispatch directly if you have taken the time to enter this information in your phone. If not, do not delay, and simply dial 911. Help will be on its way... (September 2018)
Dr. Eric Weiss