Desing for VoIP

Which could be the right design, for an enterprise with star design with 14 branch offices connected to the main office? The most important resource in the WAN is the bandwidth, for this reason make an IAX trunk between the Asterisk box in the main office and the Asterisk box in each branch office are one of the best choice for preserve this resource. But in the meantime in the LAN network of each office use SIP instead of IAX, because SIP is more common with other manufacturer. The average number of user for remote office is 200 and in the main office almost 1000. The real problem is from the point of view of the application layer, so we want to know real experience about designs like that,
How the people in one of the branch offices could dial to another office? Using prefix, DUNDI or another way?
What other application use to make a scalable design? OpenSER?
What kind of endpoint is best, reuse the analog telephone with FXS, Digium’s card like TEXXX in FXS mode with bank of channels, or new IP Phones?
Would need the main office a cluster?
Thank you.

In the remote offices you could get away with one server but if it were my network I would want 2. At the main office I would go with 3-5 depending on expected loads.

I think youll find that IP phones will price out pretty cometitavely with channel banks so it would probably be best to just go IP from the start.

As far as routing ges there are many ways to do it see link below for some discussion on this.±+dual+servers

See here for bandwith needs (keep in mind you never want to plan on saturating a link so plan planty of extra)

First of all, thanks for your answer.
The 2 server in the remote office is just for redundancy or load balancing or deploy different function???
I think that DUNDI is a pretty way to make calls between Asterisk Boxes, but in this way I must to assign a server in the main office to do the work of DUNDI lookup server? Or maybe distribute the load between all the servers in the main office?
I have been trying with IAX trunk and really save a lot of bandwidth when are many calls at the same time, so I think that is the methods for this desing!!
But my doubt is about how all this will behave, because there is an application in this enterprise that grow and grow and now is a real disaster. In some places I read about Asterisk is unable to support so many user and have to use OpenSER in some cases.
I agree with you in the IP phone. I want to know if there is any type of IP phone for Asterisk that support video like the 7940 of Cisco??
Thank you again.