The carrier discovered the error June 26 and fixed it on June 29, and has been correcting customer bills since then, said the spokesman, Harry Mitchell.

An error was made in a programming edit for software affecting changes in customer calling plans, according to Mitchell.

He said the programming error first began affecting billings on June 17. Overbilling errors ranged from US$200 to several thousand dollars each, he said.

The 11,000 customers were only a fraction of the 5.3 million served by Verizon in Washington, Virginia, Maryland and West Virginia, Mitchell said.