A phantom telephone tax has been striking thousands of cell phone subscribers in Howard County since August, and a major wireless company has confessed to the overbilling.

The extra charge -- identified as a county tax -- has been appearing on the monthly bills of an estimated 15,000 Howard cell phone users, typically adding $4 to $5 to the total. The problem is, Howard imposes no such tax.

Complaints to the county's Office of Consumer Affairs prompted an investigation, and yesterday, Cingular Wireless acknowledged making the errors and pledged to credit the accounts of everyone who was overcharged.

Stephen D. Hannan, Howard County's consumer affairs administrator, said his office has looked at a number of bills, and that some of them list an 8 percent county tax. "We are attempting to find out now how widespread this is," he said. "We're getting cooperation" in correcting the bills, he added.

The problem came to light when a resident complained to county Budget Director Raymond S. Wacks, who knew that although Baltimore City and Baltimore and Anne Arundel counties all have local telephone taxes, Howard does not.

"Clearly, we don't have the tax, and we don't have the authority to levy it," Wacks said. Prince George's County imposed a telephone tax this year.

Bill Theiss, a Howard County Council employee, noticed the $4.41 county tax charge on his $66.50 August cell phone bill from Cingular Wireless. "I happened to see it, and when I got around to calling them, they realized they were doing some reprogramming and inadvertently put Howard in," he said.

Kate Paxton, a spokeswoman for Cingular, said the Prince George's tax, which that county began collecting last month, caused the confusion. Some Howard subscribers have the same telephone exchange or ZIP code, or both, as residents of Prince George's County.