I have a system like that setup as part of my total phone system.
www.pbxes.com offers a vitrual voip pbx.
Using the premium service you can have 3 callback number.
You also need a voip provider for outgoing calls and making calls. I use voicetrading for example.
You need a did number as incoming line or callback number. For some countries you have to pay for others you can get them for free.
I alsways have 2 phones (my cheap china dual sim phone broke down after 2months and still waiting for replacement) I use my dutch cellphone to trigger the callback. I can a number and get a busy tone. pbxes than calls me back on my other roaming free mobile and I get a dialtone. From there I can call any number using voip at there low rates.
I also use the system to recieve calls on my did number. If someone calls the call is also forwarded to my cellphone and I can answer the call. My cost is only calling the roaming free cellphone which is around 8 eurocents for a +44 mobile.