AT&T announced Wednesday it would let its 1.8 million subscribers exchange real-time text over the Internet with AOL's 50 million users.
However, AOL blocked the move within hours, just as it did to several similar attempts by the Microsoft Network earlier this year.
"What's happening here is sort of like AT&T saying to its customers, if you're an MCI customer, you can't talk to each other," AT&T spokesman Ritch Blasi said Thursday.
Blasi said the company managed to work around AOL's block. But he conceded that because AOL has ultimate say over who reaches customers using its free Instant Messenger software, the attempt would probably be abandoned if AOL continues to resist.
Microsoft eventually abandoned its attempt to connect its instant-message users to AOL's, but is now allowing subscribers to instant message with AT&T's. Microsoft has called for an industrywide standard that would link every instant message, regardless of their company link.
AOL did not return calls seeking comment. The company has said it shares the goal of an industrywide standard but has security concerns about how other companies are linking to its messaging software.