Hi,
1. Do you have a listener (listen=) for all necessary IPs and transports?
2. Outbound interface selection is done either via the 'mhomed' option,
https://www.kamailio.org/wiki/cookbooks/5.0.x/core#mhomed
or, if it's not doing what you want, by manually steering it via $fs:
https://www.kamailio.org/wiki/cookbooks/5.0.x/pseudovariables#fs_-_forced_so...
-- Alex