It was found that freeswitch performs multiple INVITES. But why?

One reason why multiple invites are sent out is if FreeSWITCH doesn't receive anything back. This could mean the far end is down for some reason or that there is a network issue between FS and the other endpoint. NAT is a common issue in this sort of configuration. I recommend that you look at SIP traces on each end of the SIP dialog.

-MC