Verizon has overbilled Fairfax County by more than $3 million for phone service since 2006, and despite regular complaints, the billing problems remain so routine that the cash-strapped county has had to assign personnel to monitor its phone bill.

The phone company has already refunded $3.1 million to the county after overbilling it for services, but the county's Department of Information Technology just wrote Verizon to identify 97 additional billing errors costing $190,000.

"Clearly this concerns me," said Supervisor Pat Herrity, R-Springfield. "But looking at the bigger picture, I'm shocked at the fact that they continue to make the same mistakes. You'd think they'd be able to get it right."

Verizon has tried to fix the problems causing the overbillings, but the errors continue. As recently as July, Verizon inflated the county's monthly phone bill by more than $80,000, a county audit shows.

Most of the overcharges -- $2.5 million of the $3.1 million since 2006 -- result from Verizon consistently billing the county for services above their contract rates. Most notably, Verizon has been charging a "dial tone tariff," a fee charged to maintain a voice line, of $15.60 per phone line instead of the contracted rate of $12.

Verizon also erroneously charged the county $343,000 in late-payment charges it didn't owe, $154,000 for long-distance calls that were never made and $101,000 in unauthorized third-party charges that were made by people not affiliated with the county government.

"The County Attorney's Office is currently reviewing what recourse, if any, the County has to ensure that Verizon complies with acceptable billing practices," the county's audit stated.

Until Verizon can correct its errors, the county will assign staff to monitor its phone bill, Herrity said.

The county's contract with Verizon expires in 2014, and county spokeswoman Merni Fitzgerald said the ongoing overcharge problems will be taken into consideration when the county considers renewing the contract.

The billing issues are in addition to the problems the county has had with Verizon's oversight of the county's 911 centers. The emergency phone lines failed during a destructive storm this summer, leaving more than 2 million people unable to reach emergency personnel.

Verizon spokesman Kevin Irland said the company works closely with Fairfax County to address concerns, and will continue to investigate and resolve any issues with bills when adjustments are warranted.

"The good news is they've cooperated during this process," Herrity said. "Now they just need to get their bills right the first time."