Outbound Gateway hang up at 30 seconds with no answer

My users are complaining that SOME (not all) outbound calls ring 4 times, then go to busy signal. The FS log shows connection and status "entering state [early][183]" then 21 seconds later "Originate Failed. Cause: NO_ANSWER", then hang-up. It should at least go to the receiving ends voice mail. Why hang-up? Why busy signal? Bridge string is: bridge(mailto:{sip_from_uri=4104440002@64.136.173.30,sip_from_user=4104440002,sip_from_host=64.136.173.30}sofia/gateway/voip-innovations-outbound/14432997994) FreeSWITCH version: 1.10.2-release-14-f7bdd3845a~64bit (-release-14-f7bdd3845a 64bit) Any ideas would be a great help. Regards, Sean