From europe, the games that worked for me via Dial-up without dreampi was:
-Planet Ring (The server was updated recently, not sure if works now in that way)
-Starlancer (not tested from my part, others played it).
Each one with some "needs" like a vmu save file, a CB code or a boot/patch disc.
The main problem is how permisive is the isp that your dreamcast dials. Like Pc's said, very few allow Dns join.
I was lucky using the same for many years in the same Dreamcast, with a free ISP (i think forgotten by some company). I love to play this way because is like old-school, at the dreamcast time. Is what you wanted to do at kid but cant because was expensive, alwais watching how many time are conected.
By othe way, i have other dreamcast with a setup readu to play Online with dreampi. It needs a line inducer, is the bad part, needs some router setup too, but for the rest is a +:
More online games
Easy to use.
Less configs/ways to get online.
A bit of more speed than the original Dial-Up.
Your phone line is available when you are playing.
Dreamcast Now service.
Use it via wifi...
Like you see, Dreampi is a next steep on the Online DC scene, a big next steep