[Freeswitch-users] server never sends "183 ringing back"

Hi all, Over the past two weeks all of a sudden Vitelity has been sending me TONS of call failed notifications, but I am seeing the calls hit my server and ring and they always drop at exactly 0:30’s if the called party doesn’t answer. I opened a ticket with Vit and they said that I’m not sending back “183 ringing back” to them. How can I send them this 183 ringing back? Thanks!