their voice clearly

What could be the problem? It seems it's not the problem of the DSL speed
since I have upgraded twice and it doesn't improve at all. I used a web DSL
speed test (server located in Los angeles, I am in San Diego) to check my DSL
speed and mine is normal, and checked my parents' computer DSL speed in
China, the result is 100KB Down and 300 KB Up. So it looks like their upload
is good, then why I can't hear their voice clearly even I have an excellent
download speed on my end? And because their Download speed is poor, however,
they can still hear my voice without problem. I am just totally confused
here.