Incoming calls ring only once

Does it only ring once, or is it that it rings loudly once and then the volume is reduced for subsequent rings? If this is the case, then go to Settings>Face ID & Passcode. Scroll down to Attention Aware Features. Turn that off. This feature will reduce the volume of the phone ringing and other notification once you look at the phone and the TrueDepth camera recognizes that you have looked at the phone and are aware of the notification.

If it is only ringing once and is going to voicemail, check Settings>Phone>Call Forwarding and see if that feature is turned on. If it isn't and you are still only getting a single ring, contact your carrier. They handle call forwarding and other call handling features. There may be an incorrect setting at the carrier level to conditionally forward your calls.