Callback requests at random ports in WSO2 EI 6.0 - wso2

Question

I have a proxy to an external web services being called by a schedule task in WSO2 EI 6.0. I can see the request being sent out of the ESB as an instance gets logged in ESB Analytics but it's not clear to me what is happening with the response payload.
These log messages show up in wso2carbon.log file:
TID: [-1] [] [2017-05-08 18:22:27,875] WARN {org.apache.synapse.transport.passthru.SourceHandler} - Connection time out after request is read: http-incoming-85 Socket Timeout : 180000 Remote Address : /XX.XX.XX.XX:35380 {org.apache.synapse.transport.passthru.SourceHandler}
TID: [-1] [] [2017-05-08 18:17:27,896] WARN {org.apache.synapse.transport.passthru.SourceHandler} - Connection time out after request is read: http-incoming-84 Socket Timeout : 180000 Remote Address : /XX.XX.XX.XX:57507 {org.apache.synapse.transport.passthru.SourceHandler}
TID: [-1] [] [2017-05-08 18:12:28,364] WARN {org.apache.synapse.transport.passthru.SourceHandler} - Connection time out after request is read: http-incoming-83 Socket Timeout : 180000 Remote Address : /XX.XX.XX.XX:50387 {org.apache.synapse.transport.passthru.SourceHandler}
XX.XX.XX.XX is the remote address where the webserices are hosted. I can understand from this the the response is getting lost because these ports are not open in the firewall hosting this WSO2 instance, but for security reasons I cannot open all ports so these random ports would always work.
I need to understand why WSO2 is expecting the response at these ports and not on the same port where the request connection was made (443 in my case), and even why a separate connection need to be established since this is a synchronous web service.
Please let me know any resource available containing more details on how the transport pass trough is affecting this.
If this is some functionality from WSO2 or Synapse is there a way to disable it?
If I end up having to open all ports in the firewall to support this, is there a way to limit the random port numbers to an specific range?
Thanks in advance.