Well it looks like you can sign up online by clicking on the ATTIVA button on screen from the
www.187.it website (the International option for Teleconomy shows under the Offerta then Tariffe per Risparmiare tab).
There is also bound to be a way to do it over the phone also but my Italian isn't up to telephone calls yet ;)
I would imagine you need to be logged in as a customer at 187.it.
I recall it was easy to register (I did so in order that I could view and pay my phone bills online by credit card from England and it has worked very well)
Hope this helps
Lesley