Tellabs has become at least the fifth telecom equipment vendor to warn of lower-than-expected fourth-quarter financial results.

Last Friday evening, the company announced it expects to report fourth-quarter revenue between $455 million and $470 million. Previously Tellabs was expecting between $525 million and $550 million.

The company attributed the shortfall to “lower than expected revenue from some large North American customers, an unfavorable shift in product mix, revenue deferrals related to a new product launch, and a lower effective tax rate.” The problem in North America stemmed from uncertainty surrounding carrier mergers, Tellabs said, echoing the complaints of several vendors about delays in regulatory approval of the AT&T/BellSouth merger. Tellabs chief executive officer Krish Prabhu publicly lamented the situation in late November. Tellabs expects factors impacting its fourth-quarter revenue to be temporary, relieved in the new year by demand for more bandwidth.

In warning of an end-of-year shortfall, Tellabs joined other vendors Carrier Access, Redback Networks and (perhaps most notably) Motorola, all three of which issued warnings last week. Adtran issued its own warning in December. While Adtran’s woes, like Tellabs’, may have been caused by the AT&T/BellSouth merger, it’s unknown to what extent the same is true Carrier Access, which complained of lower sales to wireless carriers. (One such carrier, Cingular Wireless, is co-owned by AT&T and BellSouth and reportedly paused some spending while the merger was delayed.) Redback blamed its lost revenue on disruptions caused by a proposed acquisition by Ericsson. And Motorola pointed to a bad mix of geography and products in its wireless devices business.

In early December, UBS Investment Research lowered its expectations for Tellabs’ fourth-quarter revenue to slightly more than $530 million. But even that estimate has now proven to be too high.

Tellabs fourth-quarter 2006 revenue will be down 10% to 13% both sequentially and from a year earlier.