That's correct.
H323 is a peer to peer protocol just like SIP. You configure dial peers(pots/voip) for new DIDs and forward the number of digits you want to the callmanager. On the Callmanager you can play with those digits (significant digits, translation pattern..etc) to route the call the way you want.
MGCP is a master/slave protocol. CUCM being the Master agent. You configure translation pattern for new DID OR if your Phones DN matches the last digits of your DID, you use "significant digits" on the gateway config page (4 would mean the last 4 digits will be forwarded to the CUCM which will route the call directly to your phones, without the need of translation pattern).
HTH