[Asterisk-Users] 5 Channel / Trunk ??

[PBX]

To all Asterisk guru's...

Here is my question.  

1. Asterisk PBX - 5 Trunks / Incoming lines
2. 1 Building - 3 Companies (sharing phone system)


Ok that's the basic layout.  Here's the low down - Each company will
have one dedicated channel for there company.  The other 2 channels they
want to be set aside as rollover's (rotary).  That is not an issue.
Where my concern is, how can I specify if they call Company A and
primary line is busy and it rolls over to one of the spare lines, how do
I designate which Auto Attendant is played.  If it was certain channels
set aside for certain companies that would not be an issue.  

Any one have any ideas on how to accomplish this.  Is there a way to
look and see where the call was originally designated for.

Thank you,

-gcc