Simply put, all the commands you can use are in the Standard Rockwell/Conexant (originally written/invented by Hayes) command set.
M0L0 literally tells the modem to turn off the volume (M0) and set the volume control to minimum (L0), otherwise the screeching when connecting was causing all the dogs in the neighbourhood to come running.
There are thousands of init string combinations, imossible to type them all here, and I do have a couple of links.
Normally, one would not have to use ANY extra settings, but there are a few that can be used for specific troubles.
If you try the modem, and DO have a specific issue with it, post here, and glc, Sarge, or I (and a couple of others) will find you an init string for it.