When doing semi-carrier services with 1000s of DIDs or greater, freepbx gui going to cut it. Asterisk is totally fine with this config, but Freepbx, which is what Elastix uses, was not designed to handle this in an effective manner. Freepbx is a PBX, just as Asterisk is, not a carrier grade softswitch, blah blah blah...
That being said, I have a customer running a custom Freepbx with more then 30K DIDs inbound on a single sever. I'm not happy with this, but thats what the custom can afford now, but runs great with a bit of customizing outside of Freepbx. I have build shell/php scripts to help parse files that are managed through the GUI, and write to the extensions_custom.conf on a reload.
They also have tons of telemarketing spam/spit inbound, and use the Freepbx Blacklists to manage this.
It works well. I hope you have done some homework before attempting to peer with carriers like read the book on Asterisk:
www.asteriskdocs.org/ for starters. There's too much for one to mention in a single forum on carrier services like High Availability clusters and such, but with all that, I'd like to wish you good luck, and welcome to the forum.