1) Do you ever plan in the future to use an IP phone?
2) Which type of ATA are you using? The ATA-186/188 can allow one call at g729 codec and one call at g711 codec, but not 2 g729 codecs.
3) what type of medium are the remote sites going over? Microwave, landline, satellite, etc?
If you are using a centralized model you might want to go with CallManager to make configuration and control a lot easier.
I have a simular deployment to what you are looking at.
We have multiple PRI's coming into 2 VGWs set-up as H.323 gateways. Both VGW's point to the CCM cluster. The call manager is set-up with Locations to provide the CAC for each of the remotes. We utilize IP phones (conference, speaker, and low end), ATA devices, FXS ports off routers, and remote routers connected to small PBXs.
Originally the system was designed with FXS and PBXs and then we moved to Call Manager based and added voicemail with Cisco Unity. Once the customer saw the difference with IP phones they really liked them. The ATA's are used Fax machinces at the remotes. Beware to use fax on ATA-186/188 you have to set line 2 to G711ulaw.
From the reading of their product this is an itme that would work with H.323 Gateway and Gatekeepers. In CallManager there is a menu option to set-up a H.323 Gateway. adding the IP address, CSS, and so forth. However, CallManager would probably be to hefty of a price tag for the connection of the DirectWay system.
The way the documentation reads DirectWay offers the Gateway and Gatekeeper already for your system.
If they based it on H.323 then Cisco will work fine for the Gatekeeper, of course that depends on how close to protocol standard Hughes followed.
As for H.323 configuration on Cisco follow this link:
Scaling for CallManager (CCM) would not be a problem depending on the type of CCM. With 600 remotes and two ATA's at each remote that would make 1200 devices total. According to the SRND for "Cisco Unified CallManager 4.x" you can only use 500 devices such as gateways. for a centralized solution. If you are talking distributed you can have 500 per cluster. see the link.
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