Starting May 15, all four major US carriers will be implementing the new Text-to-911 program. This new way of asking for assistance via SMS aims to address situations where it would be difficult to use the usual 911 voice call.
The system also partially tries to cater to a demographic, made up mostly of very young smartphone users, who are more comfortable, not to mention efficient at sending an SMS than making a call. There are also situations that call for discreetness or silence, when speaking out loud with an operator would prove to be dangerous, if not fatal. Thus, Text-to-911 was born and will be adopted by US carriers such as Verizon and AT&T, with more to follow if they choose to join in.
The process is quite simple though not exactly as fast. You send an SMS to 911, clearly indicating your situation and location. For best results, it would be a wise idea to type in straight and understandable English if possible. It isn't a one-off message though, as an operator will reply to you asking for further details. Hopefully, you would have your device on mute for those clandestine scenarios. At the moment, only plain text-based SMS is supported, so no chance of sending photos via MMS. The latter would probably be a bit more useful in attaching a location or evidence.
Text-to-911 isn't foolproof, universal, nor is it a replacement for the tried and tested 911 call. Though the FCC approved of the new system, it reminded everyone that voice calls are still the best way to get emergency help. This is also due to the fact that even among supporting carriers, not all areas are covered. If you try to send an emergency SMS in a location that doesn't support the system, you will receive a reply informing you of the failure to deliver the message. The FCC has provided a list, which you can view from the link below, of jurisdictions where Text-to-911 will be in effect.