asterisk users - Jul 2005 - Which mix of VOIP services do we need?

For an Asterisk based "virtual attendant" application that works  with
local
inbound calls (in multiple markets) from PSTN phone lines to a  local 
DID....i.e...Seattle PSTN line call comes into a Seattle DID, or a Phoenix  PSTN
line
call comes into a Phoenix DID....so all of the activity related to the  
incoming call is "local" at this point.
 
....with call then routed to remote hosted Asterisk server...wherever we  
prefer to set it up....not necessarily in one of the markets where we have  
DID's. Incoming call is matched against the call forwarding configuration in
Asterisk for each unique DID...
 
....call is then routed back out to PSTN line in the same local market  where 
it originated.....i.e...the Seattle call originates from PSTN line dialing  
into a Seattle DID, routes to Asterisk server in remote market, routes back out 
 to different Seattle PSTN line in accordance with the call forwarding  
configuration that the Seattle DID owner has chosen.
 
So...there are no outbound calls occurring from the local DID's....as  the 
only outbound call leg occurs at the Asterisk server. 
 
I need some help. Based on total anticipated incoming minutes of 500 per  
month/per DID....and on a scale of 500 DID's total....and a target of $6-7
per
month in total costs per DID (including VOIP access per minute fees)...
 
....and working on the premise that just as we developed our own Asterisk  
virtual call attendant, we would prefer to build our DID / VOIP  access solution
by piecing together the components in the absolute most  cost effective way 
(as opposed to paying $14.95 or so for a "Vonage type"  solution)....
 
What are the types of service providers who provide each of the following  
pieces in more of an "a la carte", "build it yourself and
save" manner?:
 
---local DID's (for now just in Denver, Phoenix, Seattle)
 
 
---VOIP access for the inbound/outbound legs coming into and out from the  
Asterisk server
 

---setup for our Asterisk server (should it be colo'd and if so, what  
services are most important to ask for from the colo provider who would be 
handling
our specific solution), understanding that the cost of this is in  addition 
to the target total price per DID stated above
 
 
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