The number showing on telephones with Caller ID was 931-5425, the city's utility billing department.
Joanne Hall, public information officer, said the city's phone vendor has determined that these calls were not originating from the city of Pleasanton telephone equipment.
"The calls have been traced to a Northern California company that claims to have inadvertently entered a wrong number into their system, said Allen Hammond, Pleasanton information technology manager.
Hammond said the calls have been stopped, thanks to a block put on by AT&T on Tuesday and contact with the company in Northern California. He said information about the incident has been forwarded to city management to decide what steps, if any, should be taken against the company.
"We apologize for any inconvenience this has caused," Hall said.