Wrong Numbers : Phones: A computer glitch caused GTE to overbill as many as 1.7 million customers an average of $2 to $3. The company is not sending notices, but will credit bills later.

Ever get a phone bill so large that you’re sure it could not be right because you couldn’t possibly have talked that long to that many people, only to discover that the bill is correct and you were wrong?

Well, GTE California customers, take heart. This month, you may be right.

The phone company serving 3 million customers statewide--including most of the Westside, South Bay and Orange County--said a computer glitch caused the overbilling of as many as 1.7 million customers statewide, nearly all in Southern California.

The overbilling involves so-called ZUM calls, usually eight to 12 miles away from where the call is made, according to Larry Cox, a GTE spokesman. A glitch in the computer software billed those calls as more expensive toll calls during a four- to six-week period in March and April.

Cox said the average customer was overbilled by between $2 and $3. However, at least one Westwood customer said his business-phone bill was overcharged hundreds of dollars and that his home phone was overbilled by about $10.

Ron Schnell, who runs a computer software company called Secure Online Systems, said he normally would not have looked at his business-phone bill except that a friend asked him about his average monthly bill.

He said he discovered the overbilling and called the phone company. He said he was upset that the company is not planning to notify customers.

“It annoyed me that it wasn’t publicized,” he said. “There are probably lots of people who will pay their bills without even knowing they are being overbilled.”

GTE’s Cox said the company is not sending corrected bills because of the high cost involved, but he said that the company will determine how much each customer has been overcharged and will credit customers beginning June 7 with the overcharge amount plus 3% interest.

“We apologize for the inconvenience, and we’re asking for a little bit of patience and understanding,” Cox said.

GTE has 460,000 customers in the Westside, 160,000 in the South Bay and 200,000 in Orange County. Cox said that most customers in those communities were overbilled only if they made ZUM calls.

Cox said the cause of the glitch has not been determined, nor has the company estimated the total amount of money that was overbilled.