The way 'spying' works is, you only do it voluntarily once. You can bet your house the Chinese recorded, verified, and documented every call Milley made to them, as did all other intelligence services who happened to intercept it. After that first call, China (and possibly Russia and a few other countries) had Milley over a barrel.
Within a couple of weeks the Chinese would have approached him in some way and would have told him you will now do what we tell you do and you will tell us everything we want to know. If not, we will release the tapes.
Was Milley wearing a wire for the Chinese in his daily briefings and meetings, including with the President? Did he plant bugs here and there? That is a very real possibility. That is something foreign intelligence typically does when they snag a patsy. Milley may have been a friend of China, but it is guaranteed that China was NOT a friend of Milley.