Are you trying to do a “load” test? There are PLENTY of tools out there. You can also get your vendor to run this for you.
Do you just want to hear what it sounds like? Try ruinning Netmeeting or Messenger at each end and start a voice conversation.
I’m currently using Quintum products in our organization (D2400 on a T1 and A400s using DSL or Cable Modem).
We’ve upgraded the remote sites to use 256k upload speed. Quality is crisp (better than a Cell phone), bandwidth use is minimal, and installation was a breeze!
You can probably get a reseller (look at http://www.quintum.com) to run a “proof of concept” for you for a free 30-day pilot…I did, It worked EXACTLY as represented.