[Users] multiple 487 created

For the normal cancel situation. UA1 OPENSER UA2 --------INVITE-----------------> <-----------100----------------- --------------------INVITE----------> <---------------100------------------- ----------CANCEL----------> --------------CANCEL-------------> <--------------200------------------ <------------487------------------- --------------ACK-----------------> <---------487--------------- -------------ACK------------> But in my case, I found that there are many 487 created from openser to UA1. The multiple 487 may cause a call drop in the following call. Why does openser create so many 487 for UA1? How can I prevent it? UA1 OPENSER UA2 --------INVITE-----------------> <-----------100----------------- --------------------INVITE----------> <---------------100------------------- ----------CANCEL----------> --------------CANCEL-------------> <--------------200------------------ <------------487------------------- --------------ACK-----------------> <---------487--------------- -------------ACK------------> <---------487------------- -------------ACK------------> <---------487--------------- -------------ACK------------> <---------487------------- -------------ACK------------> <---------487------------- -------------ACK------------> <---------487------------- -------------ACK------------> <---------487------------- -------------ACK------------> ...