asterisk users - Sep 2005 - Multiple SIP Phone Calls Overlapping on the Same Phone

I have another freaky situation that is occuring. I am getting
overlapping phone calls on the same phone.

Basically when a user hits the hook fast enough, the current SIP phone
call connection does not get killed by the Asterisk server while a
second SIP connection gets created giving the user overlapping calls.
The way to reproduce this is by hitting the hook fast and furious. The
telltale sign of overlapping connections is when you get an echo from
the Asterisk voice and or static caused by the multiple prompting. What
could be causing this and how do I solve this. Here is the SIP config.

[general]
context=teleScribe
port=5060
bindaddr=0.0.0.0
allow = ulaw
allow = ilbc

[sip]
type=friend
fromuser=sip
host=dynamic
nat=no
dtmfmode=rfc2833

[sip-nat]
type=friend
username=sip-nat
fromuser=sip-nat
host=dynamic
nat=yes
dtmfmode=rfc2833

The phone used is a Grandstream Budge Tone 102.
Asterisk version 1.0.6.

I am unwilling to update the version unless I know for a fact that the
new version will correct the problem. Somehow I believe this is a
configuration problem.