Assuming your network includes remote sites located across an IP WAN from where your cluster resides, you can use the locations feature of CM for Call Admission Control. You may also want to implement a low bandwidth codec, for example, a G.729a or a G.723, for calls that cross the IP Wan using the regions feature in CM. This feature will allow you to use G.711 for calls that don't cross the IP WAN but use a relatively large amount of bandwidth (64k) and use a lower bandwidth consuming codec for calls that do transit the IP WAN. By using a lower bandwidth codec you'll be able to permit more simultaneous calls across the IP WAN without saturating your WAN capacity.
Keep in mind that one of the functions of a Gatekeeper is Call Admission Control (CAC). Since your network only has 1 CM cluster, all phones are registered with the same cluster and therefore, when you use the locations feature which allows you to specify where your phones are, the CM cluster will be aware of how much bandwidth is being used at any time. So, the CM cluster itself can handle the CAC function. However, when your network consists of multiple clusters which are remotely located from one another, you should use a Gatekeeper.
