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

The projection represents a near doubling of total subscriptions to its various services - cable TV, high-speed Internet, and voice - from the 27 million Comcast 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 to 10 percent before the introduction, Comcast officials told a gathering of analysts and investors today at the Kimmel Center in Center City.

Comcast Chief Executive Brian Roberts also said the company wasn't 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 hasn't released any results from those tests yet.

Comcast has a partnership with Sprint that would allow it to add a cell product. The cable company is also 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 its estimated $5.7 billion in capital expenditures in 2007 would bring in more revenue, instead of just dragging earnings down. They said that about 75 percent of that figure is a variable expenditure for installations and other costs related to adding services. The company expects a 25 percent return on capital expenditures this year.