Sonus Networks Inc., a Westford company that makes Internet network hardware and software, said Tuesday that it has agreed to buy Network Equipment Technologies in a transaction valued at $42 million.

California-based Network Equipment Technologies, which is also known as NET, provides enterprise-class networking solutions in the field of unified communications. So-called unified communications systems tie together e-mail, phones, and other tools over Internet networks.

In a press release, Sonus said the proposed acquisition is consistent with its strategy of transitioning its focus to cloud-based unified communications.

Under the agreement, Sonus said it will pay $1.35 a share for Network Equipment stock, a price that represents a premium of 14.4 percent to NET’s closing share price of $1.18 on June 18.

The intended transaction is expected to close in the third quarter of 2012, and it has been approved by the boards of directors of both companies, Sonus said.

As a result of the acquisition, Sonus added that it expects incremental revenue of about $15 million to $20 million in the second half of fiscal 2012.

In a statement, Raymond P. Dolan, Sonus president and chief executive, said, “We believe that acquiring NET accelerates Sonus’ strategic plan to help enterprises communicate smarter, faster, and more collaboratively.”