In my eyes this is a big hole in the windows phone app ecosystem; I thought I read somewhere that there was a SIP client though, but with some limitation on which hardware it could use so voice always came through on speakerphone?
There are no SIP clients that are useable. There is OctroTalk which was built for WP7.x. WP7.x SDK doesn't allow access to the earpiece speaker so it only works on speakerphone. The problem with that app is that everything is proxied through their servers. Virtually impossible to use reliably in NZ.