Comcast Corp. said yesterday that its digital voice service would help the Philadelphia company reach 53 million subscription units by the end of 2009.

The projection represents a near doubling of total subscriptions to Comcast's various services - cable TV, high-speed Internet and voice - from the 27 million the company had in 2003.

Comcast has 24 million customers, but totals subscriptions to its three product offerings. The company credits the projected growth to its "triple-play" marketing, which bundles the three services for customers at an initial $99 per month.

Average quarterly revenue growth since the introduction of digital voice in the first quarter of 2006 is 12 percent, compared with 10 percent before the introduction, Comcast officials told a gathering of analysts and investors at the Kimmel Center in Center City.

Comcast chief executive officer Brian L. Roberts also said the company was not sure whether it would add cell phone service to turn its triple play into a quad play. The company is testing a cell phone addition in Boston and Portland, Ore., but it has not released any results from those tests yet.

Comcast has a partnership with Sprint Nextel Corp. that would allow it to add a cell product. The cable company also is part of a consortium that bought $1.4 billion worth of wireless spectrum at a government auction last year.

"We haven't figured out a compelling reason to add wireless, other than that it sounds nice," Roberts said.

Company executives also sought to reassure investors that Comcast's estimated $5.7 billion in capital expenditures in 2007 would bring in more revenue, rather than just dragging earnings down. They said about 75 percent of that figure was a variable expenditure for installations and other costs related to adding services. The company expects a 25 percent return on capital expenditures this year.