Many thanks Emoci for the tip. It works like sharp!
I have a vague memory from about over a year ago. I tried to register on Sipbroker a Sip address, also from a non-supported network. For some reason it didn't work then, and I was left with an strong assumption that aliases could only be created for networks which already exists on their list - just for people who want to hide their Sip addresses
That's only why I didn't think of giving it another try... (in other words: I'm not that negligent)
Thank you Josef too for contribution on this post.
Yes it's possible to define outbound Dial Prefixes on LIV, but I don't think it's applies for callback numbers too (on their callback section you need to tell them which provider you want to perform the callback, so that's seems separate from all other dial-rule configurations). Anyway the website is in Italian things are more difficult to figure out.
SipSorcery looks excellent, esp with it's added new features, but at this point I'm experiencing many times audio problems. They have just started and I'll wait some time before happily use them again, together with Voxalot