sip to sip calls overlapping..!

[rolandah]

hello all,

i have the following topology:

<Softphone> -> <Asterisk> -> <sipura3102>

whenever i recieve a cal from sipura through asterisk, or make a call through asterisk and then out of sipura.. sometimes sip to sip calls are over lapping... in other words: softphone > asterisk >sipura is overlapping with another softphone's phone call and the two calls gets merged into one with every1 hearing the other...
if one of the SIP users hangs up! all other SIP users on the same asterisk have their line break as well..!

any idea what might bthe cause of this?!

please find attached my messy extensions.conf and sip.conf

thanks for any advice..