Fail over trunk

Hi there ,

Can any one tell me suppose for routing one trunk only for a country (other country routing is good) in case for some reason it could be busy or congestion or for something failed can switch to another terminator
that we know as fail over trunk but i dont know how will i configure this.
can any body help me with this? a2billing has option for fail over trunk but it seems dont work or may be also configuration not known.
But i want to know how to do it in asterisk level.

And also came to my mind in what perspective we will say this trunk is failed i mean is it for congestion to that country or may be particularly for that country line i broke?

Thanks in advance.