WASHINGTON — All Americans may soon be able to get help from 911 just by sending a text message.

The service is available in only certain areas and on major national wireless carriers. But under new rules approved 3 to 2 Friday by the Federal Communications Commission, all cellular service providers will be required to support text-to-911 universally by year’s end.

“Our first responsibility is to provide for the safety of Americans,” said FCC Chairman Tom Wheeler, “and this is a step to continue to fulfill that responsibility. And it is not a final step.”

About 400,000 Americans contact 911 daily, according to the FCC — and about 70 percent of them do so with cellphones. In places where it’s too dangerous for a caller to speak, or when the caller simply can’t talk, text messaging could be a lifesaver.

Text-to-911 comes with unique challenges, though. Unlike calls from a cellphone, where getting accurate location data is a difficult but surmountable problem, it’s unclear how text messages will convey location information without the user disclosing it by hand. Dispatchers may not know where to send help in response to a texted plea to 911 — at least, not to the same level of specificity that a call from a landline can provide.

Republican FCC Commissioner Ajit Pai said the FCC’s vote encourages people to “dive in . . . when in fact there’s hardly any water in the pool,” saying that even if wireless carriers complied with all the regulations on time, only a tiny share of 911 dispatchers — about 2 percent — currently accept text messages.

The FCC’s regulations will also apply to third-party messaging apps, such as Google Voice and Apple’s iMessage, which are increasingly popular among cellphone users. Requiring the apps to support the feature may be a complicated process.

One question is how the FCC will determine which third-party messaging apps will be covered by the rules. Only apps that “interconnect” with text-messaging systems — meaning that they’re set up to send messages to phone numbers as opposed to simply routing messages across the app’s own systems — will be subject to the rules. Some officials, such as Republican FCC Commissioner Michael O’Rielly, say that putting 911 location requirements on software developers would lead to consumers’ privacy settings being overridden.

Some worry that an eventual shift away from text-messaging technology altogether could undermine the regulation.

“Even the applications that have integrated with SMS, like iMessage, only fit the definition so long as the SMS technology remains in service,” AT&T wrote in a blog post Thursday. “Once that technology is retired, those apps no longer fit the definition.”

AT&T itself anticipates a shift away from traditional text messaging.