http://web.wenxuecity.com/BBSView.php?SubID=computer&MsgID=168860
Now it looks like
ATA <--> SipSorcery <--> Sipgate, Voipdiscount
The out-plan is
ATA --> SS --> VD (to international destinations), or GV (for free out-going US domestic calls)
The in-plan is
ATA <-- SS <-- SG <-- GV (free US in-coming calls, and cheap callbacks to China in-coming calls)
Only one line of the two lines of SPA1001 is needed.
I will see if I can write a detailed documentation for above configuration.
Cheers.