At about 7 p.m. Tuesday, the man reportedly knocked on several doors in the 2400 block of Cambridge Drive and claimed to be offering a deal for elderly residents, police said.
The man had an AT&T identification card with a photo, but the photo was believed to have been of someone else, police said.
The man asked residents for personal information, which he claimed was part of the sales procedure.
Police contacted AT&T and confirmed that the man was not an employee of the phone company.
Residents are being advised not to give out any personal information to the man and to call police if they see him.
Anyone with information about the case is asked to call Antioch police Detective Kollo at (925) 779-6895.