1. Take a look at this link:
http://www.mysipswitch.com/forum/viewtopic.php?t=1054, it might help. But I have trouble to forward my calls, so it is not working for me.
2. You also can try to run Sipsorcery local version. The good: FUP exceeded issue doesn't exist any more when call China, and if sipsorcery.com is down for some reason you still you phone service available. The bad: call number in US I have to hang up phone first, then after 10 seconds I get connected.
So far nothing is perfect, I am still searching solutions. I think fitwxc can help us on these issues if he is not too busy.
Good luck.