Third place U.S. wireless carrier Sprint Nextel may introduce unlimited voice service for as slow as $60 per month to counterattack a wave of new plans charging $99, analysts say.

On Tuesday, the largest U.S. carrier, Verizon Wireless announced an unlimited price plan of $99 for voice calls. Rivals quickly followed, with AT&T and T-Mobile USA making similar announcements.

Investors in Verizon and AT&T saw the value of their companies' shares plunge. The drop shows a real concern that both companies will have to make bigger price cuts, Bear Stearns analyst Phil Cusick said in a research note to clients.

We believe the stock sell-off implies that Sprint will undercut very aggressively and that AT&T and Verizon will eventually be forced to respond, he wrote.

Cusick noted that a price of $75 for unlimited calls would only put Sprint at a slight advantage over its rivals.

If instead Sprint prices closer to $60 for unlimited voice, that would create much more marketing stir and be a real differentiator, but would risk an eventual response from competitors, he added.

UBS analyst John Hodulik expects an unlimited plan between $60 and $80. He said Verizon and AT&T's share price drops assume an $80 Sprint plan. He added that Sprint could me more aggressive.

He said he doubted that AT&T or Verizon would match the plans but that could hurt their customer growth and revenue.