I hate to say it, but WM is leading here.
Awful as Windows Mobile is (truly it is, I'm a freetard on all my other devices) it seems to currently be the only choice if you want a wide range of apps, not those chosen by the vendor. Android will fill that role eventually, I hope, but it's too young to have the range of apps just yet.
The problem with WM and VoIP unfortunately is that the earpiece isn't easily accessible by developers, however they're slowsly finding their way on this issue. I believe Skype supports the earpiece on certain OMAP devices from HTC, and Agephone (a good SIP client) supports it on HTC OMAP devices, the Touch Diamond, Touch Pro, and the SonyEricsson Xperia X1. Possible the Touch HD too, I forget.
Aside from that earpiece issue there aren't any restrictions, all the VoIP clients can use 3G. Whether your 3G connection is up to it is another question, mine has the bandwidth but it's not stable enough for SIP really, and at times has awful latency. Annoyingly whenever *I* test it it's fine, but when I need to make a call to someone it's invariably awful and I end up having to make the international call from my mobile as normal :(