Long distance Voice Quality Issues

[kukar]

I am using freephoneline.ca long distance service to call overseas. But most of the time the call quality is not very good, the callee on the other side hears echo and has a hard time understanding. My setup is as following Asterisk Server with multiple local extensions (spread worldwide), sip channel from freephoneline.ca. I don't have any issues with calling anyone on my local extension. I use ulaw and alaw as the codecs in that order. Any suggestions.