Can you ellaborate on how you have your VoIP system setup?
What Codec are you using for audio compression?
The greater the compression, the greater the likliness of “chopping” (jitter). G.711 & G.729 are usually fine, but going to G.723 or G.726 (greater compression) will increase latency leading toward jitter. As James Mentioned, Silence Suppression (VAD) can sometimes add to jitter, but I reccomend leaving it on unless you really have bandwidth to spare (haha, imagine that).
Sherman, The fact that you are connecting over a 56k line and to the public internet is probably the main source of your problem. In order for VoIP to be very effective and have any kind of really toll quality or close to it, you need guaranteed bandwidth. When dealing with the public internet, you have no QOS in place, and you may also not getting the voice packet priority queing which is essential for VoIP to work snmoothly and clearly.
My suggestion is to either call the carrier and discuss these topics with them to see how they handle VoIP, or try getting an ISDN line, DSL line, or Cable modem to give you more bandwidth. This should help alleve the problem, but probably not entireley elliminate it.