The minimum bandwidth is going to be dictated by your RTP codec and voice signaling protocol you are using. It being a softphone is irrelevant unless you account for HTTP services overhead as well which is really accounted for in most IPT implementations.
Here is a good document as well as link to the Voip Bandwidth calculator.
http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a0080094ae2.shtml
http://tools.cisco.com/Support/VBC/do/CodecCalc1.do
Please rate any helpful posts
Thanks
Fred
