Directory Integration with a Multi-Forest Environment is supported in CUCM through Active Directory Light Weight Directory Service (AD LDS).
Once AD LDS is configured, the configuration on Cisco side is very similar to doing a regular LDAP integration on CUCM.
AD LDS runs as a service on one of the Domain Controllers. In the picture below, the AD LDS could be installed on any of the DCs as long as that DC can speak to the other DCs.
A good configuration guide can be found here: