Nico,
http://www.mywebcalls.com offers callback service called ANI Callback.
You register callback number on their web site, then you call their number (which is always bussy), and they call you back, giving you dial tone to continue dialing.
They charge you for call to you (Canada) + they charge you for call to your destination. I used them a lot before Voipbuster appeared.
I have tried to initiate development of such product on this and various other web sites, but no luck so far. I event though of programming something like that to myself, but I hope someone else (like Voxalot) will do that for us. It would be great tool.
Cheers from Montreal