Hi Claude,
Check the Voip Bandwidth calculation tool here:
http://tools.cisco.com/Support/VBC/do/CodecCalc1.do
You can play around with the impact of different voice payload sizes vs overhead to determine the resulting voice call bandwidth.
Note that IPSec overheads can be up to 58 bytes, which when added to the IP/UDP/RTP and Layer 2 overheads can have a significant impact for each 20 byte voice payload packet.
Increasing the voice payload size can make quite a difference.