UCM has no direct concept of DIDs. The DNIS pattern (commonly a DID) for an inbound call at a gateway is optionally transformed and then searched for within the gateway or trunk CSS. If a match is found, the call is directed to that destination.
If you purchase additional DIDs from a provider you just need to ensure that the gateway/trunk is properly configured and the number is reachable from the CSS of the gateway.
This assumes the gateway is configured to communicate with UCM for the call. If you have an H.323 or SIP gateway, you'll need to ensure that the dial peers are correct in addition to the UCM-side configuration.
