Not much time to write on my blog these times, anyway: I found this interesting article on how to make free/cheap calls from web browser and I regret to notice that Sitòfono is not listed there.
But I guess this is due to the "free" model that has been considered: free calls that nobody pay, except the VoIP company holder of the free web telephony product.
BUT this is not a sustainable model, it needs to gather some revenue stream, which is not VC funding!
On the other hand, Sitofono is based on a more realistic revenue model: Sitofono is bought by companies/professionals who want to be reached from the web by various media channels (email, chat, voice call). Caller does not pay for the call, which is offered by the sponsoring company/professional.
Just one more point: one of the disadvantages of web call back model is the possibility of making call tricks or jokes to some victim. In this case, the offender puts the number of the victim in one of the boxes, and starts the call. The offender is not immediately recognizable, unless by asking the VoIP company who started the call.
There is no "perfect" answer to this, but one strategy is better than none :)