I had a bit of a play and fring does looks promising.
Fring + VoXaLot provides least cost routing to the mobile phone.
I have posted on the fring forum for them to consider this combo. See:
http://www.fring.com/forums/showthre...=1962#post1962
I would encourage other VoXaLot users to post also as I really do see this as a powerful combination.