According to https://www.kamailio.org/wikidocs/cookbooks/devel/core/#mem_join the default value for mem_join is `1 (enabled)`.

[https://github.com/kamailio/kamailio-wiki/blob/main/docs/tutorials/troublesh...) says:

In order to merge the free memory fragments one should enable memory join. Default is disabled (mem_join=0).

So depending on the source of information the default value of mem_join is different.