The position of defence minister is a civilian position. If the tables were turned, it's absolutely appropriate for the Chinese defence minister to contact the American equivalent.The US SecDef doesn't just 'cold call' the Chinese Defence Minister. There would've presumably been preliminary communiques between the two sides at the departmental level on the nature of the phone call before leadership at the Chinese MoD then decides whether to accept the American request for a ministerial call.
All that is to say China most certainly already knew what the phone call was about. If it's a cross-border civilian matter, which this is, it ought to be civilian government institutions that deal with said matter i.e. between the US State Department and China MFA, not Lloyd Austin telling Wei Fenghe to pick up the phone and be like: "Dude, what's with the balloon?"
It makes no sense for China to refuse a phone call. America is the country that has been intent on exacerbating a Chinese-American conflict.
Think of it this way....why are the Americans telling us this? Why didn't they keep quiet about it like they did with Russia? They want to push the narrative by not cooperating the Chinese has something to hide.