Tried this and it works.

1/3MB /min using G729, 1.2MB /min using uLaw. Incoming calls are always uLaw with my VOIP provider because of how they originated, but calls I make are always G729. It seems to work great except when switching towers it has a break in the upload and you can hear them but they cant hear you. That and connection time (a bit over a second) are the only things they could improve on, I still couldnt recommend it for business use but that could be my VOIP provider. Push works well.

SpecialKen about Groundwire: VoIP SIP Softphone, v1.6