Which of the following are functions that are often provided by an ITSP but are not typically  provided by the PSTN？（） A:  QoS B:  call setup and teardown C:  call routing D:  call supervision E:  audio signal compression

Which of the following are functions that are often provided by an ITSP but are not typically provided by the PSTN？（） A: QoS B: call setup and teardown C: call routing D: call supervision E: audio signal compressio