When a phone is answered you hear a busy signal

this happens 1 out of 7 calls. When the phone rings and you answer you hear a busy signal. The caller is disconnected. We opened a ticket with our carrier thinking it was them and this is what they came back with. Please keep in mind the users are not behind a firewall. Any help would be appreciated. Thank you.

Ticket Updates :
12/11/2009 10:19:00 AM – by AriS
As noted in the below progression, we receive a delayed call confirmation from you, followed by a BYE request. As indicated earlier, this is typically caused by port/session filtering such as those found in a firewall.
12/11/2009 10:17:00 AM – by AriS
GP NOC has located a problematic call in the capture:

274.174786 64.136.174.30 -> 216.170.98.242 SIP/SDP Request: INVITE sip:14124065070@216.170.98.242:5060, with session description
274.676736 64.136.174.30 -> 216.170.98.242 SIP/SDP Request: INVITE sip:14124065070@216.170.98.242:5060, with session description
275.677801 64.136.174.30 -> 216.170.98.242 SIP/SDP Request: INVITE sip:14124065070@216.170.98.242:5060, with session description
277.677468 64.136.174.30 -> 216.170.98.242 SIP/SDP Request: INVITE sip:14124065070@216.170.98.242:5060, with session description
281.676728 64.136.174.30 -> 216.170.98.242 SIP/SDP Request: INVITE sip:14124065070@216.170.98.242:5060, with session description
281.691648 216.170.98.242 -> 64.136.174.30 SIP Status: 100 Trying
281.692572 216.170.98.242 -> 64.136.174.30 SIP Status: 180 Ringing
283.843934 216.170.98.242 -> 64.136.174.30 SIP Request: BYE sip:+14123777711@64.136.174.30:5060
283.881608 64.136.174.30 -> 216.170.98.242 SIP Status: 200 OK