What Milley did is commonplace. From the retired Commander of the US Army in Europe:
Milley's contacting foreign general officers, like General Li Zuicheng of China, is a big part of his job. In these communications, I'm confident that the Chairman followed the protocols for engagement with foreign officials.
Why do I believe that? Because as a senior general officer commanding in a foreign theater, I often had to conduct these types of phone calls and engagements, as do all generals or admirals in similar strategic circumstances.
Engaging with foreign military commanders -- whether they are allies, partners, friends or even foes -- is a major and extremely time-consuming requirement for senior military commanders. We do this to strengthen alliances, build trust, prepare for coalition exercises -- and to stay in contact via informal communication channels during tense times.
When I was Commander of the US Army in Europe, there were 51 countries in our area of responsibility, and my visits and contacts were conducted as part of an approved plan for "security cooperation" within that theater. Chairman Milley's job is much bigger and more difficult, because his covers the globe.
His responsibilities require an endless series of engagements with US Embassies, foreign governments, and the military leadership of more than 190 countries, even those, like China, whom many Americans consider the enemy.
Often the communication channels with foes compliment the diplomacy generated by the State Department, and this is where generals and ambassadors are critical teammates.