* Austrian unit takes accounting charge of 48 mln eur
* Q2 EBITDA 299 mln eur vs Reuters poll avg 320 mln eur
* Company trims 2014 revenue, capex outlook (Adds details on net loss, underlying results, Austria)
VIENNA, Aug 13 (Reuters) - Telekom Austria said an unexpected accounting issue overshadowed improvements at its Austrian business and pushed it to a bigger than forecast loss in the second quarter.
The former state monopoly, which was taken over by Carlos Slim's America Movil last month, said new accounting software revealed it had overestimated unbilled revenues in its Austrian fixed-line business.
This led to a one-off adjustment to turnover of 48 million euros and a negative effect on net profit of 28 million euros, in the second piece of bad news since Slim launched the takeover offer that brought him a 51 percent stake.
Telekom Austria, which Slim wants to use as a platform for expansion in central and eastern Europe (CEE), wrote down its Bulgarian unit by 400 million euros in June, while the takeover offer was in progress.
The writedown and the accounting problem pushed it to a net loss of 359 million euros in the quarter from a profit of 52 million euros a year earlier.
The company's shares were little changed in early trading in Vienna.
The company trimmed its full-year guidance, saying it now expected 2014 sales to fall by about 3.5 percent instead of 3 percent, and capital expenditure in the range of 650-700 million euros instead of 700 million euros ($936 million).
Telekom Austria is active in seven CEE countries: Austria, Bulgaria, Croatia, Belarus, Slovenia, Serbia and Macedonia.
Second-quarter earnings before interest, tax, depreciation and amortisation (EBITDA) fell 9 percent to 299 million euros, while revenue fell 8 percent to 963 million euros, missing the average estimates in a Reuters poll.
The declines were worse than in the first quarter, when Telekom Austria had begun to slow a profit decline with cost cuts and tariff hikes in its main, domestic market. EBITDA last quarter fell 5 percent and revenue 7 percent.
Excluding the accounting charge and other one-off effects, Telekom Austria said it would have had an EBITDA increase of 6 percent in its Austrian business, despite losing 5 percent of its mobile customers due to the tariff increases.
It is trying to focus on its high-value customers and make them pay more for the increasing amounts of data they use.
Telekom Austria shares, which have been buoyed by Slim's 7.15 euros-per-share offer that shareholders may still accept, were barely changed at 7.14 euros in early trading. (1 US dollar = 0.7478 euro) (Reporting by Georgina Prodhan; editing by Tom Pfeiffer)