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* Pre-Call announcement with B2BUA Application Module *
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Description:
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The annoucement server plays a wav file to the caller.
It searches for files to play following these rules:

 1) [AnnouncePath]/[Domain]/[User].wav
 2) [AnnouncePath]/[User].wav

Then it connects the caller to the r_uri in Back-To-Back 
User Agent (B2BUA) mode, thus staying in the signalling path 
of the call.