I have been researching this myself. If this functionality could be done, with it would likeluy bring a huge potential for toll fraud. Without some sort of authentication it wouldn't be very sensible far as I can see so far.
What i did do for a few people working from home is set them up with a line on their phone that has call forward external capabilites to be used for forwarding. They access their user web page, and change the "forward to" number to whatever umber they need to call, local, national, inernational, etc. Then they only pay for a local call to work and from there callmanager takes over and extends the call elsehwere. Using account codes so that's the "authentication" and tracking to prevent fraud (call reporting will uncover any issues with fraud, plus cant make the call without a valid code).
(only good for someone in front of a PC tho, on the road on a cell phone it wouldn't be doable unless they are calling the same number each time)