I've seen a lot of chatter on the web from people having some questions about using their dial-up Tivo with the Ooma service. From my personal experience--which includes an ancient Philips series 1 Tivo, as well as a series 2 Tivo--the solution is fairly simple.
Go into Tivo setup, Phone setup, and add *99, as the dialing prefix. You may want to disable call-waiting also (you get that free with Ooma!), so enter *99*70, as the prefix instead.
I've also found that some dial-in numbers seem to work better than others, so if you are still having trouble, try a different number. Even if it is long-distance (free with Ooma!)
Some folks say adding a DSL line filter helps, but I've had varying degrees of success with this, and right now do not believe it to be necessary. Shouldn't hurt though.