Hi Jame. Thanks for the answer.
But what I was trying to do is to avoid buying new gear.
I've found a way to authenticate my Issabel SIP Trunk like it were an extension on SOPHO PABX.
The 'peer details' field on Issabel's trunk setup should be something like:
username=0005 //// this is my SOPHO extension
secret=****** //// my SOPHO extension password
qualify=yes //// or 'no'..whatever
host=192.168.0.5 //// my SOPHO's IP address
fromuser=0005 //// again my extension on SOPHO PABX
fromdomain=192.168.0.5 //// my SOPHO's IP address
allow=alaw,ulaw,g729,gsm //// codecs I need to use