The four major cellular providers -- AT&T, Verizon, Sprint and T-Mobile -- volunteered to participate in an Federal Communications Commission program that lets people contact their local 911 emergency services via text message. The deadline for getting it all set up and working is today, so the service should be available as of now in places where a 911 call center is technically prepared to receive texts.
The experts still recommend that you use voice 911 calls, because they can provide emergency centers with much more information. But people who are deaf, hard of hearing, have a speech disability or may find it otherwise dangerous or impossible to voice call 911 will greatly benefit.
The service required upgrades to local 911 centers, but as of today it should work from any iPhone (or any text-capable cellular phone) on any of the four major carrier networks. A big caveat is that while the cellular companies have added the feature, your local public safety officials have to sign on to the service before it can be implemented. Some will opt out due to costs, but it looks like the service is coming up rapidly state by state and locality by locality. In most cases, the 911 center getting your text won't have location information, so that should be in the body of the text. Voice calls to 911 generally provide a call center with your approximate location. Also, users are cautioned that text delivery might incur small delays in delivery, and that voice calls will still be faster.
If you have any doubt about the availability of text-to-911 service in your area, check with your cellular provider. Please don't test this, but cellular companies say they have implemented a bounce-back feature notifying senders of 911 text messages if they were not received for any reason.
You can find more information in the FCC FAQ.