proxy - How to make "poor man's roaming" by proxying cell calls to SIP or Skype?

What applications should I use to make the Android device to forward calls from cell network to SIP or Skype and back?

One device ("Home") is expected to be left at home, serving as proxy; other device ("Travel") is just using data connection to connect to the first device, so it works like this:

1. Someone is calling you using cell network.


2. Home device receives the call and calls Travel using SIP.



3. Travel device sees the incoming SIP call.


4. You answer the call.


5. Home device is proxying between SIP and cell network for you until you drop the call.

How to do it? How to workaround greedy cell providers?