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Valid CCNA or any CCIE Certification can act as a pre-requisite.

The Cisco Certified Network Associate Voice (CCNA® Voice)

640-460 IIUC Implementing Cisco IOS Unified Communications (IIUC)

Or

642-436 CVOICE 6.0 Cisco Voice Over IP (CVOICE)

The fowling excert is taken from the Cisco Website

The Cisco Certified Network Associate Voice (CCNA® Voice) validates associate-level knowledge and skills required to administer a voice networkThe Cisco® CCNA Voice certification confirms that the required skill set for specialized job roles in voice technologies such as voice technologies administrator, voice engineer, and voice manager.

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