pc-to-pc voip: unsuccessful so far

Archived from groups: comp.dcom.voice-over-ip (More info?)

I have been testing, as a newbie, some voip applications/providers.
First, Skype on both Linux and Windows, and everything was working fine most
of the time.
Then I have been trying the SIP applications.
Me and a friend of mine (in two different european locations, both with DSL)
have registered first both with sipphone.com and then both on babble.net,
using X-lite on both sides, or X-lite and Kphone, or Kphone and the babble
software.
And in all cases, although we were able to establish a phone connection, the
voice quality was not enough good to have a conversation (we could hear
some vocal sounds, but nothing else).
In the case of babble it is pretty strange, as I am able to make phone call
through it (using X-lite or Kphone, DSL or even an analog modem) to normal
telephones. But I have never succeeded in making pc-to-pc calls.
Can anybody suggest me how to solve this strange thing, or at least how to
go in the right direction to solve it?