Hi,
Voice streaming data needs a guaranteed bandwidth available to ensure a good quality conversation.
G.711 codec (64 kbps voice bandwidth) would require 84 kbps of network bandwidth while G.729 codec (8 kbps) would require about 24 kbps network bandwidth.
I don't have any experience with GSM but I think the voice bandwidth is almost the same with G.729, around 9.6kbps.
The additional bandwidth is required for the extra RTP (Real Time Protocol) header for every voice packets.
VoIP Quality is affected by delay and jitter.
Here is a link that explains about
VoIP Quality of Service
The break up is most likely because of the ISP in China does not have QoS setup for voice traffic.
My suggestion is to use low bandwidth codec like G.729 or GSM to minimise the bandwidth requirement.
Have you tried to call using a direct IP Address by dialling
sip:xxxxxx@A.B.C.D
where
xxxxxx is your Voxalot number
A.B.C.D is your IP Address from your ISP
If you use a router, you may need to setup the port forwarding to forward TCP 5060 to your PC or ATA adapter.
Just my 2 cents.
Hope this helps.
Adrian