AT&T Inc said on Thursday that 40 percent of its iPhone sales since the beginning of the year have been to business customers, suggesting that the phone is encroaching on an area dominated by Research in Motion's BlackBerry.

AT&T also generates almost half of its total revenue from business customers, said Ron Spears, the head of the company's business services, at an investor conference.

The company would not say how many iPhones it sold during that period.

Analysts estimate that AT&T, which started selling iPhone in 2007, has about 15 million iPhone users or 17 percent of its total customer base. RIM reported a total customer base of about 41 million for the BlackBerry in its most recent quarterly report.

Comparing those numbers is difficult, however, because AT&T serves U.S. customers, while RIM's customer base includes the United States and other countries.

This includes wireless and wireline services and would imply business revenue of almost $62 billion revenue for 2010, based on analyst expectations for 2010 total revenue of $124 billion, according to Thomson Reuters I/B/E/S.

In 2009, the company said that its wireline service revenue from business customers was about $40 billion out of total revenue of about $123 billion.

AT&T shares were up 41 cents or 1.7 percent to $24.54 in afternoon trading on the New York Stock Exchange.

(Reporting by Sinead Carew. Editing by Robert MacMillan)