From the OCS AD Guide:
msRTCSIP-Line
This single-valued attribute contains the device ID (either the SIP URI or the TEL URI of the phone the user controls) used by Microsoft Office Communicator for telephony. This attribute is marked for Global Catalog replication and is indexed. In Office Communications Server 2007, if a user is enabled for Enterprise voice, this attribute stores the E.164 normalized version of the user’s phone number.
msRTCSIP-LineServer
This single-valued attribute contains the SIP URI of the CSTA-SIP gateway server. This attribute is marked for Global Catalog replication but is not indexed.