I talked to a guy in Sweden over the Internet and the call was clearer than my cell phone.
The call was placed at a SuperComm 2002 demo booth from a SIP phone to another SIP phone at the headquarters of a company called HotSIP, which makes SIP server software. They guy on the other end sounded like he was RIGHT THERE, and the delay was less than you get on a typical cell call or even some traditional international calls.
The Internet service provider on this end was Level3. The phone was connected to the Internet via an ethernet link to an OC-12 fiber ring in downtown Atlanta, so there was probably very little delay or packet loss on this end. I don't know what the story was in Sweden.
It was just one demo (and who knows where that Swedish guy reallly was) but it belies the notion that you must get high-priced guaranteed levels of IP service to do intelligible IP voice calls. The call I was on was good enough to transact business with a partner and certainly good enough for an in-company phone call. Especially an international one, which can be expensive.
You might want to check out a cheap way to test voice over the Internet to see if it could be useful to you.