Tellabs ( TLAB) shares tumbled 11% Thursday after the company cut its third-quarter guidance. The Naperville, Ill., telecom equipment maker posted an adjusted profit of 16 cents a share on $549 million in sales during the second quarter. Those numbers compare with pro forma earnings of 13 cents a share on $463 million in the year-ago quarter. The solid performance exceeded analysts' estimates, which called for an adjusted profit of 14 cents on sales of $541 million, according to Reuters Research. But looking ahead, the company was less optimistic. Tellabs told analysts on an earnings call Thursday that telco consolidation, particularly the pending merger between AT&T ( T) and BellSouth ( BLS), will put a drag on third-quarter sales. Tellabs now expects third-quarter revenue to be about $522.5 million, well below the $543-million target analysts had in mind. The company's shares fell $1.26, or 11%, to $9.74 in early trading Thursday.