Sonus Networks , which makes voice and data communication equipment, posted a surprise quarterly loss as longer deal cycles took its toll on gross margins, sending its shares down 8 percent after market.

There were certain deals that we previously expected to recognize as revenue in the second quarter but were pushed to the third quarter because acceptance requirements were not completed in time, Chief Financial Officer Wayne Pastore said on a conference call with analysts.

Had these deals closed in the second quarter, product gross margins would have been about 70 percent, Pastore said.

Sonus, which counts AT&T and CenturyLink unit Qwest Communications among its top customers, saw product gross margins fall to 67.7 percent from 70.9 percent a year ago.

The company, however, reiterated its full-year revenue forecast of $265-$285 million.

For the foreseeable future we expect our business will continue to be lumpy due to the cyclical buying patterns and general large deal sizes from our service provider customers, Chief Executive Ray Dolan said on a call with analysts.

The company reported a net loss of $5.9 million, or 2 cents a share in the second quarter, compared with income of $377,000, or break even, a year ago.

Revenue for Sonus, which competes with Acme Packet Inc , fell 15 percent to $51.8 million.

Shares of the Westford, Massachusetts-based company were trading down 10 cents at $2.64 in trading after the bell. They touched a low of $2.53 immediately after the company posted its results.

(Reporting by Siddharth Cavale in Bangalore; Editing by Don Sebastian)