[Asterisk-Users] asterisk box after an analogic pbx

[Accursio Avona]

Hi all,

I'm newbe with asterisk and i'm facing with this problem that i'm not 
able to solve.
I've to put an asterisk box after an analogic pbx wich require a 0 digit 
to give the dialtone.
So when a client ask asterisk to dial an extension it should

1) send the 0 digit
2) wait for the dialtone
3) dial the extension the client send.

How can i obtain this result?

Thank's in advance
Best regards
Accursio Avona.