I have been involved in designing a few networks, but you need to be more specific in describing the setup, number of subscribers, number of minutes, is it postpaid or prepaid. One or 2 stage dialling. Are we talking retail or wholesale. Will the gatekeeper need to be in control of the call, or letting it go after the callsetup. Will it need H323 firewalling to hide your networkstructure from partners.
If unsure take any old ciscorouter and give it an GK IOS, that will get you going.
Addie you need to be more specific..