AT&T and US WEST Communications have identified a billing problem potentially affecting customers enrolled in an optional AT&T long-distance calling plan, both firms said.

Errors in US WEST Communications' billing system beginning last July may have caused AT&T customers enrolled in the Reach Out America plan to have been incorrectly billed.The mix-up potentially affects 280,000 customers in Utah, Arizona, Colorado, Idaho, Montana, New Mexico and Wyoming, a spokesman said.

The total dollar amount of the billing errors has not been calculated, but in only a few cases will it exceed $20, the companies estimated.

AT&T said customers affected by the problem will be notified by mail beginning in October. US WEST is examining all affected accounts and will begin to make any bill adjustments with the October billing cycle.

AT&T pays US WEST Communications to provide the billing in its 14-state region for AT&T long-distance services under an agreement reached after the 1984 Bell System divestiture.

Calls to US WEST and AT&T have increased because of the billing problem, and the companies ask that customers be patient when trying to reach these offices. Customers who have questions after they receive information in the mail should call the AT&T service office number on their bills.