United Online (NASDAQ:UNTD) and its hearty 6% dividend have piqued my interest for quite some time. I haven't invested in the company, and probably won't, because the dial-up side of its business is likely to die fairly quickly, and I'm not yet convinced its other offline businesses will grow fast enough to offset that decline.

A similar argument can be made against regional Bells like Motley Fool Stock Advisor pick SBC (NYSE:SBC). Can the Bells boost sales of DSL and other network services quickly enough to offset the decline in voice revenue? Interestingly, the two stories are intertwined. The faster the Bells ramp up DSL, the worse things get for United Online's dial-up business.

The Bells' DSL has two problems, though: its ability to reach dial-up customers and its cost in comparison to dialup. The same issues also apply to Comcast (NASDAQ:CMCSA) and its broadband Internet service, though cable's reach tends to be better.

This morning on the train, I saw a couple of Verizon (NYSE:VZ) advertisements that indicate the environment for DSL may be changing. Verizon's latest offer of DSL access for $14.99 per month, and the ongoing pricing battles between cable and DSL, would seem to end the economic argument that dial-up service saves money. That still leaves the problem of reach, but the Bells are improving in that respect as they continue to heavily market DSL.

Investors interested in United Online are left with the non-dial-up portion of the business, which includes subscription services like classmates.com and other advertising and commercial revenues. These are profitable businesses, but I have very large doubts that they can grow substantially enough to offset the company's just-about-guaranteed loss of dial-up accounts. At the end of the day, a large part of dividend investing is the sustainability of the dividend -- this Fool isn't convinced that the free cash flow that covers the payout is sustainable.

Dial up further Foolishness:

Nathan Parmelee has no financial interest in any of the companies mentioned. The Motley Fool has an ironclad disclosure policy.