Nextel Communications Inc., trying to prevent its cell phones from interfering with emergency response broadcasts, is offering a consortium of phone carriers and public safety groups $350 million more to reorganize use of the nation's radio frequencies.

The Reston wireless phone carrier yesterday committed $850 million, up from $500 million proposed in August, to a proposal before the Federal Communications Commission that would untangle crisscrossing radio signals that can leave police officers, firefighters and ambulance drivers with nonworking radios. The local blackouts are temporary but their frequency has grown with ballooning cell phone use.

Nextel has worked with local emergency agencies that report trouble with cell phone interference to try to alleviate problems in specific areas. But a comprehensive reorganization of the bandwidth Nextel and other carriers share with emergency workers is necessary, they say.

The costs of actually switching emergency responders off Nextel's frequency will likely be more expensive than originally thought.

After a detailed analysis of the three-year plan, Nextel decided to bolster its contribution, company spokeswomen Audrey Schaefer said. An FCC spokesperson was unavailable for comment last night.

Nextel, the fifth-largest carrier in the United States with more than 10 million subscribers, operates its wireless network mostly in the frequency range of 700 to 900 megahertz. Fire and police departments also use the 800-MHz frequency, which sometimes mixes with Nextel's signal.

The funding for the FCC proposal is contingent on the commission approving the plan to reorganize the 800-MHz band. All cellular providers must buy FCC licenses for radio frequencies, which the agency controls. Once the plan is approved, Nextel said the conversion will take about 42 months.