Hey Bigdog,
Yes, this is definitely something you can do -- Voxalot does this and much more...
You've already got a voxalot account setup, correct?
Setup your IPKALL # to go to this voxalot account
Setup your softphone to login to voxalot
Softphone Configuration Settings - Voxalot FAQ
Setup fonosip as a Voice Service Provider
Configuring Provider Settings - Voxalot FAQ
That should be it, unless I'm forgetting something... If all you want to do is use Voxalot as incoming from IPKALL and outgoing for fonosip, you will be fine with a Basic account (it also includes voicemail)
Good luck... Write back if you have any problems...