Phantom subscribers continue to haunt Cingular.

For the second time this year, the SBC ( SBC)- BellSouth ( BLS) wireless joint venture has had to revise its closely watched user statistics.

Atlanta-based Cingular admitted this week that it had overstated its first-half subscriber count by 154,000. Corrected numbers show Cingular, the nation's No. 1 cell-phone carrier, with 51.4 million subscribers at June 30. The move comes just nine months after Cingular reduced its reported fourth-quarter gross-subscriber gains.

A Cingular representative atrributed a large portion of the latest adjustment to a miscount of AT&T Wireless customers switching to Cingular contracts. He characterized the discrepancy as "a small variation" that represented just 0.3% of the customer base. But some investors aren't as eager to give the company the benefit of the doubt.

"That's 15% of their net subscriber adds in the second quarter, the way I see it," says one New York hedge fund manager who had considered buying SBC until he saw the Cingular announcement Monday.

Cingular doesn't see it that way at all. The rep said the latest adjustment, which had no bearing on second-quarter financial performance, reflects miscounts over the three quarters reported since the merger closed. That means about 154,000 new users in a period over which the company reported adding 4.5 million.

The Cingular rep declined to comment on what impact, if any, the change might have on key measurements like customer defections, or churn, and average monthly revenue per user, or ARPU.

July's initial release of first-half subscriber data showed a gain of 1.07 million users on a net basis during the second quarter. At the time, Cingular's gain struck observers as a disappointment , given the 1.4 million net new users added in the prior quarter and the 1.25 million that analysts were expecting.

With about two-thirds of people in the U.S. having cell phones, the competition for subscribers has heated up. Cingular and its rivals -- Verizon Wireless, jointly owned by Verizon ( VZ) and Vodafone ( VOD); Sprint ( S); and Deutsche Telekom's ( DT) T-Mobile -- all have posted strong growth this year.

But skeptics say the industry's growth numbers aren't as pure as you may assume. Some practices -- ranging from tallying dead accounts to double-counting users -- can muddy the telcos' true subscriber growth and user loyalty performance.

Back in May, reported of growing concern about dirty numbers in the wireless business.

One whistleblower, a person familiar with Cingular, said in May that in at least one region, Cingular was counting AT&T Wireless customers who switched to Cingular as new subscribers. Cingular strongly denied that claim at that time.

Earlier this year, Cingular revised its fourth-quarter gross subscriber additions, cutting about 200,000 customers from its original 5.7 million tally. That reduction may seem insignificant, but the move helped the San Antonio wireless giant trim its churn to 2.4% from 2.6%.

At the time, Cingular blamed a lot of the difference on "conformity issues" between how it defines a customer and how its merger partner AT&T Wireless did.

For example, Cingular found that 81,000 AT&T Wireless accounts were nonexistent. About 65,000 of those so-called customers were actually expired prepaid accounts that were never taken off the subscriber rolls. Cingular also cleaned out 12,000 AT&T Wireless customers that the company said had two lines.

The Cingular rep said the second-quarter reduction cleans up the AT&T Wireless migration count.

"We have resolved the issue. We foresee no more adjustments," the rep said.

Observers point out that Monday's adjustment could make the company's third-quarter numbers, due out next week, look better by comparison. But for some investors the doubts only multiply.

"You don't know what to believe," says the hedge fund manager. "It looks pretty scummy."