There are many ways to forward your calls to your cell phone overseas. I'll describe one of those ways here. Feel free to suggest other methods in the thread below.

I setup my US cell phone to forward my calls to a Skype Online Number. When a call comes in to the Skype Online Number, it causes an incoming call popup on my computer if skype is running. In the skype software on my computer, I also setup call forwarding in the menu: Skype / Account / Call Forwarding. I enter in my Thai cell phone number here. If you don't pick up the call on your computer, it will eventually ring afterwards on your international phone. If your computer is turned off or skype is not running, your calls will still be forwarded to your phone!

Note: Sometimes it takes quite a few rings for skype to dial your cell phone instead of expecting you to pick up on the computer. I believe you can shorten the time it takes for skype to switch to your cell phone in the menu. Otherwise, the people calling you may not let it ring long enough for the call to ring on your cell phone.

Bonus: You can save money by not buying a Skype Online Number if you use Google Voice on the same US phone number your skype account is registered to. Instead of setting up a Skype Online Number, setup a Skype To Go number. This is free. The number you configure with the Skype To Go number is your international cell phone. Skype then gives you a US based phone number as a shortcut to dial that international number. Then you setup google voice to forward your calls to the US based Skype To Go number. When that happens, Skype To Go thinks Google Voice is you trying to dial the international number. It will then ring your computer and your international cell phone anytime someone calls your Google Voice number. Be sure to setup Caller ID on Google Voice to be your original US phone number

Post your methods for forwarding calls overseas, or feel free to ask me to clarify what I'm talking about above.