Until last month, the most obvious reasons for network disruptions were natural disasters like hurricanes or floods. Now, though, Verizon and other telephone companies must worry about the possibility of physical attacks on their installations. Mr. Babbio warned last week that significant harm could be done to the nation's communications system if terrorists destroyed the 50 or 100 most important central offices. Verizon, which is the dominant telephone company on the Eastern seaboard and operates in 30 states overall, is seeking to increase security at its central offices, where it is required by federal law to lease network access to its competitors. After Mr. Babbio issued his warning last week, competitors said they would resist tighter security measures if it made it more difficult for them to conduct operations within Verizon's central offices.