Line Speed of Modem

How can I directly regulate the line speed of the newer modems? The earlier modems did this by the S37 register, the the new modems no longer support this. Anyhow it seems impossible to get a detailed list of modem commands.
The reason this is needed is because in third world countries the modems often connect at too fast a speed for the telephone lines they are running on. As the consequence there don’t work very well. If they connect at a lower speed all is fine.
Incidently, changing the speed in the windows modem setup doesn’t seem to help.