Refrain from using the cisco gatekeeper. Indeed it is a regular Cisco router (like 26xx in example) running a special IOS with gatekeeper features. But it is a crap…
There are a plenty of powerful gatekeepers (both free and for a fee): GNuGK, AquaGK, MERA, Alter PSS, Quintum call relay…
what do you mean under realtime server?