VoIP BW Calculations

Once again I was down in grass trying my hand at bit level VoIP calculations rather than relying on a calculator and I found myself in a quandary about one way vs. two way calculations. If you follow conventional wisdom and estimate ~26Kbps for a G729A voice path, is the actual required BW twice that number? Seems to me that in general there is only one RTP stream flowing in one direction at a time since (again in general) people don’t talk at the same time. However, I come across instances where folks are doubling the required BW to account for this theoretical 2-way flow. So, which is it? 26Kbps (G729A), per phone call or 52Kbps? One such example is NetIQ Vivinet Assessor’s white paper “Planning for VoIP”, section “CODEC Selection”. I welcome your feedback. Thanks.