I think I have discover the problem. - Will test it out Monday on the spot.
I have test and play a lot with the machine, the RTP ports set on the gateway are different then the one I use by the settings, so I had in fact only 9 RTP ports... and on the gateway I have opened 3000 ports but on different numbers...
I just forgot to edit that part of the configuration.
So if I understand this correctly , 1 call require 4 ports ?
So active calls can be limited by :
Trunk channel limit, asterisk.conf maxcalls=255 , by SIP settings for port range.
Probably few more that I am not aware of right now