Wiki - Spy call

Hello. I apologize for writing here, but I have not found any contacts. Can you please tell me what this "Spy call" function is? - https://github.com/CellularPrivacy/Android-IMSI-Catcher-Detector/wiki/glossary-of-terms#spy-call What devices support it? Is it possible and where to read about "Spy call" in the RFC GSM specification? If there is type 0 for "quiet sms" or "ping sms", then what type of call is for "Spy call"? I searched the internet but did not find a mention anywhere. Thanks.