I am sorry, but I simply do not believe that you are so unintelligent that you cannot read 2 threads.
I will try!
How could you set that up on a multiple extension number, that wasn't even your own?
You can't! That is precisely the point I am making. If however the landline number was mapped directly to your Skype account, (without the need for dialling an extension) you could potentially trigger a callback to it, by manually registering the number with the callback service you wish to use! (e.g. By typing it into the 'Your Number' field on any of the Betamax brands which have the Phone2Phone feature). The Betamax callback triggers are generally done via the web, not via traditional CLI methods such as missed call or a SMS.
I initially misunderstood the method of this ring2skype service, as I thought there was no extension number to dial. However, as there is one, it completely rules out using this gateway for callbacks as well as diverts. However both would be possible by building your own gateway as I have outlined above. This does however have the nuisance of needing to leave a PC running at home (connected to broadband). Still for many of us, this is hardly a problem.