AT&T, the largest U.S. telecommunications carrier, on Monday said it plans to acquire Interwise, a 13-year-old voice, video, and web conferencing firm for $121 million.

The deal is the latest of three such purchases by large firms, underscoring the growing popularity of IP-based conferencing.

In March, Cisco announced its acquisition of multimedia web conferencing firm WebEx for $3.2 billion, and in August IBM said it plans to acquire WebDialogs for an undisclosed sum.

Cambridge, Massachusetts-based Interwise  has taken more than $90 million in funding from investors including Lazard Technology Partners, GE Capital, and UBS Capital.

Not long ago voice, video, and web conferencing were distinctly separate technologies that required fairly extensive user expertise to operate.

But firms such as WebEx and Interwise have managed to simplify, stabilize, and integrate multimedia conferencing on IP networks to the point where the market has grown considerably over the last few years.

“Conferencing is no longer something just for executives,” said Frank Zvi, CEO of Interwise. “Voice, video, and web conferencing is now given to everybody within the company just like email.”

Research firm Frost & Sullivan estimates that the overall multimedia IP conferencing market in 2007 will generate $5.9 billion worldwide.

The growth and ubiquity of broadband communications has also served to spur the market for IP-based voice, video, and web conferencing and to make business communications a “unified” experience.

“From an end user perspective, a more integrated unified communications solution package is easier to manage and more cost effective than buying all the applications and managing them separately,” said Elizabeth Herrell, an analyst with Forrester Research.

The Interwise acquisition, which is expected to close in the fourth quarter, will make the firm, which has 130 employees, a business unit within AT&T’s global services arm.