Comcast ( CMCSA) has taken aim at the phone business, but it's unclear when the company might make its mark.In recent years, Comcast has become both the nation's largest operator of cable TV systems and its biggest provider of high-speed Internet service. Now it's looking to fully tap into the potential of a related but rival industry. In a presentation to the news media last week, Philadelphia-based Comcast unveiled some of its still-fluid strategy for winning the phone business from the regional Bell operating companies. And, addressing some of the investor concerns that have driven down cable stocks in recent months, a Comcast executive insisted that the threat of telcos stealing cable operators' video business was overblown. The attention Comcast and other cable operators are paying to the telephone business illustrates one of the great unanswered questions for investors trying to value the cable business. Like high-speed Internet service and digital video services, Internet-style telephony represents a huge potential revenue stream for Comcast. Yet with Comcast now providing the next-generation service to only a few hundred homes, the success of such a rollout -- and its impact on Comcast's cash flow -- remains unknown. On Friday, the stock rose 4 cents to $27.87.