To all and Daniel
have you tried 1 ipaddress and different source ports?
I have not tried it myself, however folks here having the same issue have tried using both "chan_sip" and "pjsip" working on different ports of the same machine (both under control of a single Asterisk instance) and reported that there was no success - only one of the two accounts was successful on register.
If my understanding is correct the solution is simple, in kamailio use force_send_socket to send requests to the sipserver from different adresses (10.1.1.1N).
Yes, that's precisely what I was looking for!
The hard part is how you will differentiate the REGISTERS from asterisk to the sipserver.
The imaginary solution to this in my head is telling Kamailio to listen to 10.1.1.1N:5061 (N=0-4) on dummyN as well, to which Asterisk can register by using"register => 1N@ntt-east.ne.jp:<PASSWD>:001N@10.1.1.1N:5061/<EXTL_TELNUM>"
P.S.-For those who might question, the purpose of using dummyN and enabling ProxyARP on eth0 instead of adding eth0:N is to make the reconfiguration easy when addressing plan changes (which has regularly occurred in my experimental setup).