Kamailio has no way to know the difference between logically separate 'extensions' unless you tell it. Whilst each extension number may exist on multiple accounts on your PBX behind Kamailio, you'll need to find a way to uniquely identify each of them inside of Kamailio. That could be a concatenation of account ID and extension number to create unique users on the same domain, or a subdomain for each account ID - either way, if you want Kamailio to see them as unique AORs, then you'll need to establish some way of making them so.

Best,

Charles