Can you isolate the problem to any specific lights? Of course, obvious question, but if all the lights are off does it go away? Does the buzzing get louder as you turn more lights on?
It's possible that this is caused by older magnetic ballasts (which themselves typically buzz). You could try replacing just the ballasts with electronic ballasts, just be sure they are equivalent and pay attention to the difference in wiring. It may also mean that you need new bulbs, as you may not be able to find an electronic ballast with the same characteristics, but that's not maybe not terrible since T8 bulbs (1" diameter) are supposed to be better and more readily-available than T12 bulbs (1.5" diameter).
Why do the bulbs buzz when dimmed?
When you pass AC current through the filament of an incandescent bulb, it produces a magnetic field. This magnetic field tends to push against itself, so there's some mechanical force acting on the filament itself which causes it to vibrate slightly. When running at full power, the AC is fairly close to a sine wave (either 50Hz or 60Hz, depending on where you are) so this vibration is pretty much constant and generally barely noticeable.
A dimmer will truncate and clip part of that sine wave; this introduces a lot of higher-order harmonics, which in turn changes the characteristics of the magnetic field, changing the way the filament vibrates. Depending on how the filament is supported, this may cause resonance at certain dimmer settings, leading to the vibration becoming noticeable.
See this article for some more background information on how dimmers work, including a mention of this phenomenon.
Are there other bulbs that don't buzz when they're dimmed?
If you prefer incandescent lights, look for rough service bulbs; the filament and its supports are designed to withstand more vibration than normal bulbs. You could also try halogen bulbs or even dimmable CFLs.
Best Answer
For a wired phone (or the base of a cordless), it is most likely caused by having the phone wiring running directly next to electrical wiring. Since it only happens when this light is on, most likely it's the wire going from the switch to the light fixture - though it could also be caused by a run somewhere else. In general you don't want any communication (telephone, network, audio, speakers) running in parallel near by to any AC wiring, as you'll get induced hum. Always keep them at least 6" apart, or when they do have to cross, make them cross perpendicular (90 degree angle) to each other.
For an older cordless phone, it could be that your phone is operating on a frequency harmonic of 60 Hz (which is what the AC line and ballast are working on -- 50 Hz in europe). Try changing channels, and see if that makes the problem go away. Note that even with cordless phones, where the base station is connected can be subject to the same problems as any wired phone.
If it's a wiring problem, then it's best to disconnect and re-run that wire from somewhere else that isn't right next to AC wiring.
If it's not wiring, or you can't re-do wiring, one of the easiest fixes may just be to get a new cordless phone. Make sure it's a DECT phone (most are, these days) as those are frequency hopping, use digital communications, and are compatible with other wireless devices like WiFi (802.11a/b/g). You can get multi-handset cordless phones for <$100, and the nice thing is you can locate the base station anywhere near a phone line, and have a couple remote handsets with just a charger base that plugs into the wall, no phone wiring necessary.
Note that even if you go the cordless route, you may still get hum induced on the line by this particular wire, so you may need to disconnect the phone line that runs into the kitchen from the rest of your phone wiring.