Originally Posted by jackal

Most VOIP services will be blocked by the GFW, so you would need to run a VPN and then make your calls through the VPN. IME, that introduces a lot of potential for congestion and circuitous routing and will lead to mediocre call quality.

When overseas, I've actually found that GV does the best, as it a) uses the fairly robust G.711u codec and b) routes your data to the closest Google POP to you and then over Google's private global network onward to the PSTN--that is, if you are in Cape Town, your data connection is to Google's servers in Johannesburg and from there over Google's circuits back to the US, so latency and congestion are actually fairly low.

With a cross-border HK SIM, your data connection is effectively based in HK, so it'll hit Google's servers in HK with relatively low latency.

If you use GV over a US-based VPN over a China Unicom connection (say, with a Chicago endpoint), then it'll traverse the public Internet between China and Chicago (with all of the extra congestion at the GFW) and then hit Google's servers in Chicago. That's a lot of potential for jitter and packet loss and other things that are detrimental to a voice call.

So, my experience using GV overseas has actually been very good, and my experience using it with the Cross Border SIM was good as well.