SIP call initiation problem

Hi

I have problem with understanding the call setup process in the following situation:
CLIENT_A <----> ASTERISK <----> CLIENT_B

I have sniffed the call setup process and depicted in the following image what i have found out:

what is conversation marked on the image as ‘conversation3’ for??? I understand that in conversation1 the UserA tries to call UserB, while in conversation5 UserA is asked to sends sound directly to UserB, omiting the Asterisk server.

clients are configured in asterisk sip.conf file with standard options:
[client]
type=friend
context=sip
host=dynamic
secret=password

Thanks for help