The core is generated by the shutdown procedure, so not a runtime event, as side effect of running out of memory when building the transaction -- maybe some of the fields might be inconsistent. It needs to be fixed, but it is a bit hard to track it without proper core file and debugging symbols.
What are the values for shared memory and private memory (the -m and -M command line parameters)?
Did you killed kamailio or it was stopping itself?
If the later, can you be sure that each process generates a core file, because shutdown procedure can overwrite the core file that is generated at startup. You should have at least two core files in such case.