I have just moved a client over to SIP and everything went smoothly. However, a few days after the move they stopped hearing ring back when dialing to an external number. Audio is fine once the other end picks up, but there is no ring back during the call. Ring back works fine for calls between extensions. I have been working with the SIP provider and they're seeing that 3CX is declining their 183 session progress. Any ideas or solutions? They are on version 15 and the latest service pack.