Guys!!!
The problem was quarantined BUT NOT SOLVED!!!
I tested to run the 2 TRUNKS i describe in my previous post, from a Linksys PAP2T and feed the lines to my TDM400P as regular phone lines...

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I FOUND THAT THE PROBLEM ON MY ELASTIX SERVER PERSISTS!!!!!!!!!!!!!!!!!
CONCLUSION!!!
The problem is not a trunk or elastix or freepbx problem but a DSL router/modem problem!!!
If now someone can find what the heel is going on a DSL modem when from an internal network (192.168.0.0/24) you run an elastix server and on this server or on a drifferent hardware (e.g. another elastix or PAP2T device) you try to register ACTIONVOIP and SIPDISCOUNT trunks, THEN the whole SIP terminals (on the elastix server) go UNREACHABLE!!!
Regards,
John