U.S. Cellular’s parent company says it will delay issuing its first-quarter financial results until Monday after discovering accounting errors going back to 2007.

TDS found that it had overstated service revenues and understated sales tax liabilities in 2007, 2008 and 2009. Although TDS called the errors “immaterial” on an individual basis, the company said the cumulative effect of the errors “would be material to the first quarter and would impact comparisons to prior periods.”

TDS between now and Monday plans to adjust its previously reported annual and quarterly results to account for the errors. The company will also record other “immaterial out of period items,” including adjustments related to rent expense disclosed in its 2009 annual report.

TDS and its wireless division, U.S. Cellular, had planned to announce earnings today. Analysts polled by Yahoo! Finance and Bloomberg expected the company to make $0.46 per share on sales of $1.26 billion in the first quarter ended March 31. TDS made $0.64 per share on sales of $1.25 billion during the same period last year.

Shares of TDS fell over 2.5 percent by 9 a.m. Central.