I live in Saudi Arabia, and use VoIP on my mobile phone all the time. Over here, we have 3.5G HSDPA, and the networks provide upto 7.2mbps. I use gizmo project, and use the built-in VoIP (SIP) client on my phone. There's a half second delay in voice transmission at times, but otherwise, when on 3.5g, it seems perfectly normal. It's as good as a normal call.
