The service -- launched last week -- was designed to integrate a range of Telstra commercial and residential directories behind a single number, 1234.
However, Optus customers that attempt to use the service today were being redirected to Optus' international directory service, leaving it struggling with an unexpected surge in callers.
A spokesperson for Sensis said that the information service company was currently in talks with Optus to solve the problem, but late today no such solution had been found.
"Optus weren't prepared for this surge in traffic so we're working with them to find a solution for [it] whether that means diverting them back to Telstra we don't know," said the spokesperson.
Optus customer service was today politely advising callers that had reached them accidentally via the 1234 number, to call 0411 1234 to reach the Telstra-Sensis branded service.
Sensis today hid behind a disclaimer on its marketing and advertising material to defend the foul-up.
"Our disclaimer states that this service is available to all Telstra [customers]," said the spokesperson.
Sensis said it would now "revise" what the disclaimer said to ensure that consumers understood it clearly.