Some useful documents are:

Telephony Links.
The http://www.GHTROUT.com/ WWW site has links to many telephony related sites.
A Cisco document on general Traffic Engineering, with a method of calculating VOIP. It is a good starting point, but does not cover Layer-2, Control, or VAD effects. Also, Frame Relay overhead omits the ‘Flag’ byte required for each frame.
Read It.
A white paper on ATM–AAL2, explaining clearly how the multiplexed voice channels are assembled into frames of ATM cells. The VAD discussion is superficial; applying an optimistic VAD multiplier to the final answer, instead of including the effect in the number of CELLS per FRAME.
Read It.
A discussion similar to the one above, that also helps explain ATM-AAL2.
Read It.

	Voice over IP Fundamentals By: Jonathan Davidson, et al. Covered topics: The Public Switched Telephone Network (PSTN), signaling specifications (including Bell System, ISDN, and Signaling System 7), the basics of Internet Protocol (IP), modulation and compression of voice, Quality of Service (QoS), H.323, Session Initiation Protocol (SIP), and gateway protocols. Business considerations of Internet telephony are also addressed.
	Carrier Grade Voice over IP By: Daniel Collins Daniel Collins has written a terrific reference for anyone interested in understanding Voice Over IP. From the very start of the book, it is clear that Collins has the ability to address key areas of interest for technical readers, planners, and executives.
	Configuring Cisco Voice over IP By: Syngress Media, et al. This is the book that I have been looking for. The first chapter of the book provides a good high level business justification for VoIP in the enterprise and then quickly launches into a background on the technology. Intricate detail on the particulars of H.323, SIP and MGCP. The following configuration chapters also clearly presented the concepts using examples and sample config files. Highly recommended for anyone considering a VoIP deployment. VoIP will be included on the CCIE as well.

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