SAN FRANCISCO (CNN/Money) - If you're a Nextel cell-phone customer, you know why you signed up -- because of the phones' walkie-talkie feature that recalls the CB radio heyday and '70s trucker films like "Smokey and the Bandit" and "Convoy." But starting today, Verizon customers can join in on all the "breaker breaker" fun: Verizon is launching its version of nationwide "push-to-talk" service.

Verizon's (VZ: Research, Estimates) entry into the market is the first competition Nextel (NXTL: Research, Estimates) has seen for its Direct Connect feature, which has almost single-handedly pushed the company to new heights.

Nextel currently claims 11.7 million customers, 95 percent of whom use Direct Connect, according to a company spokesperson. And Verizon isn't the only company hoping to join the push-to-talk bandwagon: Sprint (PCS: Research, Estimates) is expected to announce its own launch later this year.

While Sprint and Verizon should rightfully give Nextel investors reason for concern, the new competitors face big hurdles in their efforts to steal market share and customers from the leader.

The first challenge is technical: Nextel uses a dedicated radio frequency for Direct Connect calls. Verizon's and Sprint's offerings will both route over standard cellular networks. According to analysts who have tested Verizon's service, it takes approximately the same amount of time to "instantly connect" as it does to make a regular cell-phone call, while Nextel's connection remains truly instantaneous. Don't think customers won't notice.

"The value of the fast communications goes down when it takes longer," says Keith Waryas, an analyst with IDC.

The second challenge is practical. Push-to-talk customers can communicate only with others on the same network. Nextel already has nearly 12 million customers; Verizon is starting from scratch and will have to work hard and fast to seed the market.

What's at stake is obvious: The cry from customers to have multi-network access could be especially loud from the enterprise sector, in which a business could find its suppliers using one service, its customers using another, and its workforce still another.

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Since the competing technologies use different networks, however, the arrival of true interoperability may require government intervention. Which means slow growth, most likely, for Sprint, Verizon, and other wireless carriers that jump into the market.

For some observers, the lack of interoperability gives Nextel a brief window of time to plot a strategy to protect its current lead.

"Nextel is going to need a couple of buffers, and the lack of interoperability is one," says Jeff Kagan, an independent telecommunications analyst. "If they didn't have these buffers, they'd be in serious trouble."

So keep an eye out: What Nextel does during the next few months to improve its service, change pricing, and/or win new customers will reveal a lot about whether Sprint and Verizon can make the walkie-talkie wars worth watching.

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