Abstract: A method for interworking supplementary call services between different VOIP protocols is provided. The method comprises receiving, during a call between a first end device and a second end device, a first message in a first protocol format for the second end device to transfer the call to a third end device; determining interworking information to interwork the first message to a second message in a second protocol format; generating the second message based on the interworking information, such that the second message initiates the call transfer to the third end device.