Haven't got around to producing this is in SIPp yet as I've been tied up with other projects and I'm a newbie to SIPp.
The problem only occurs when children in Kamailio is set to anything above 1 which points towards a multi-threaded issue, as it only occurs on TLS not TCP or UDP this would appear to be an OpenSSL multi-threaded issue.
As far as I can tell OpenSSL isn't multi-threaded, to use with mutli-threaded applications requires that at least two callback functions are set, locking_function and threadid_func. I searched for these functions in the Kamailio GitHub repository but couldn't find anything, is this correct? Source: https://github.com/openssl/openssl/issues/2165