integration Succession-BTS10200

Hi, we are working with a nortel pbx and a softswitch BTS10200 from cisco, but we have the following problem:when a customer dial the pbx number, automatic operator answer and ask to dial a extension, he dial the extension but nobody answer, so he hang up, but the extension keeps ringnig and if he dial the pbx number again let’s say in five minutes, the number only ring and ring but the automatic operator never answer. any ideas why this happen?