Cisco IOS 15.1 and 15.2, when the Multicast Music-on-Hold (MMoH) feature of Cisco Unified Communications Manager (CUCM) is enabled, allows remote attackers to obtain sensitive crosstalk information by listening during a PSTN call, aka Bug ID CSCtx77750.
http://www.cisco.com/en/US/docs/ios/15_1/release/notes/151-3TCAVS.html