ABCNews.com reports that Vonage Holdings Corp., the nation’s largest provider of internet phone service, and several smaller companies could be barred from signing up new customers in many markets because they failed to meet a deadline to provide reliable emergency 911 service to all subscribers. AT&T Inc. also didn’t meet the deadline. The Federal Communications Commission had given providers of internet-based phone service 120 days to comply with its order requiring enhanced 911, or E911, in all their service areas.