Abstract: Systems and methods are provided for effectively using a trunk line in a speech compression system by dynamically selecting or changing the trunk line based on a band use rate. Then the line usage is equal to or greater than a threshold value, a currently connected call is switched to a compression method which requires less bandwidth. A switch enabling attribute is set for each can and a switch enabled call is selected as a target for switching and a transmission side and a reception side acquire the resources that are needed for use of compression after switching. When the acquisition of the resources is successful, a data transfer is performed for the currently connected resources and the newly acquired resources. After the data transfer, both resources are synchronized and the resource that was used by the call are released.