[k_semler]

Windows Mobile comes with a standard SIP client, you just have to configure it via *cab is all. Yes, I can, (and do), use VoIP over HSPA, but the lag is very annoying, so I don't use it very much. (I go through skype). If you use the built in client, you can switch between your cellular number and the VoIP assigned number right in the dialing application. Until cellular data speeds, (and ping time), is drastically improved, VoIP over celluar will remain a "because I can" application of technology. A ping time of 397ms is not unheard of on a cellular data connection.