NEW YORK (FORTUNE Magazine) - Verizon CEO Ivan Seidenberg's one-time monopoly is under siege from cable companies and other competitors, who are siphoning off his core fixed-line phone customers at an alarming rate.

He's in a low-margin business that requires near-constant multibillion-dollar infrastructure upgrades -- witness the company's huge $32 billion debt load. And he needs to begin the messy process of cutting costs at MCI, which Verizon acquired for $8.5 billion in January.

More from Fortune Scandal rocks the polo set The Marchionne revolution at Chrysler The latest tech tool: People power FORTUNE 500 Current Issue Subscribe to Fortune

Seidenberg's aggressive push into broadband will give him access to 6 million potential customers by the end of 2006. If he can sign up a sizable chunk of them for high-speed Internet and video services, he may be able to keep rivals at bay.

Even under the most optimistic scenarios, revenues from broadband services are unlikely to offset declining local-phone sales.

Verizon's prized wireless unit outperforms its rivals in the pack, but mobile-phone growth is slowing.

Much of this bad news is already priced into Verizon (Research) shares, which have tumbled 25 percent from their 2005 high. Given Verizon's strong cash flow, this is a stock to own for its rich dividend yield -- now 5.2 percent -- not big price gains.