I'm helping my friend with his OBI - I can't remember but I think there was a router setting to correct the overly long huge delay in conversation going back and forth?
I think you are referring to audio latency.
http://en.wikipedia.org/wiki/Latency_%28audio%29#IP_Calls VOIP is going to have more audio latency than the public telephone system. VOIP latency is often better than cellphone latency. VOIP latency can vary, as you know.
I would suggest your friend tries try http://www.pingtest.net/
Pay attention to jitter as well as ping time. Contrast that with your test results.
100 Mb Ethernet will probably not make a noticable improvement, but it is a good idea anyway. http://www.obitalk.com/forum/index.php?topic=5454.0
If those cannot explain what is happening, describe his system.