Vonage Holdings Corp. added three investment banks to its initial public offering on Monday. The Internet telephony company said in an amended filing with U.S. regulators that Bear Stearns, Piper Jaffray and Thomase Weisel will serve on the banking team for its IPO. Citigroup, Deutsche Bank Securities and UBS Investment Bank remain the three main underwriters of the IPO. Vonage said it's making progress to provide E-911 capabilities to nearly all of its remaining subscriber lines within the year. Seventy-five percent of its lines were providing 911 service as of April 1.