This article is from the Mac communications FAQ, by Bruce L Grubb BruceG6069@aol.com with numerous contributions by others.
This varies depending on your local phone company, but often, if you
preced the phone number you wish to tone dial with "*70," (omit the
quotes but not the comma), you can disable call-waiting FOR THAT CALL
If you have a strictly rotary dial line, try preceding the
phone number with "1170".
In the United Kingdom, the code to use is #43#.
In New Zealand, the code to use is *52.
If you are using Telecom Australia,
ATDT#43#,;H Will turn call-waiting OFF
ATDT*43#,;H Will turn it back ON again.