Hi,
I was using Platinom in the past (www.platinom.net) and liked their service. No major busy signals, and 200 pesos a month. Their tech support needs some help. But if you're a veteran you should be OK.
I just signed up with Centennial (www.centennialrd.com) for a dialup account. I'm waiting for them to get in touch with me. RD$250/month and they guarantee a connection on your first attempt.
There are more options other than CODETEL and Tricom. I would never get an account with either of them because of the nightmare stories I've heard from friends and relatives.
Good luck,
Eric