[SR-Users] ADSL client-side problem

Hello, I am experiencing some disconnections with my VOIP phones. Let me explain: My server is an Asterisk + Kamailio in a datacenter with a fixed IP, and a bridge firewall in front of it, so no NAT here. My client (several offices) have usually no problem when they have a fixed IP, but encounter fatal disconnection when they have a dynamic IP. I had the case with Siemens, SNOM and Aastra phones, without any difference. They are of course natted behind an ADSL router. I also have clients doing load-balance between two ADSL lines, which is also problematic. The only way for me to recover is to change the private IP of the VOIP phones. Seems more like a TCP problem or a routing problem, but I am no network guru. I have two ADSL lines to test it, and when I switch from one line to another, every phone becomes "Not registered". Even falling back tot he first line does not help recovering. I did not try to wait for longer than 24 hours to recover. So my question is, more generally: is it possible to allow the clients (VOIP phones) to re-connect with a new IP address without having closed correctly the first connection (SIP session, I presume)? It seems not by default, but I played with the timeouts server-side and phone-side, without luck. I saw many posts with PBX having problems when behind a dynamic IP, but nothing about the clients having dynamic addresses or load-balancing. Normally, having a dynamic IP does not mean that your IP will change << on-the-fly >>, but each time you shut down and reboot later your ADSL router, but we came to the conclusion that every client NOT having a fixed IP was experiencing fatal disconnects. So I managed to test and validate this behavior with our two ADSL lines. Hope someone can help me. Greetings Simon