There are many options for integrating Cisco Unified CallManager to voicemail systems, such as digital set emulation, Microsoft TAPI, QSIG, and so forth. The vast majority of third-party voicemail integrations use the Cisco VG248 or the Digital PBX Adapter (DPA). The method of integrating Cisco Unified CallManager to the voicemail system depends on how the voicemail system is currently integrated to the PBX.
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/srnd/4x/42vmail.html