Well, for all we know that phone call has already taken place, but there's a part of me that suspects it may only happen now, in the wake of this artillery barrage.
It helps to remember WHY NK is a nominal ally of the Chinese: as a buffer state against a strong US ally on their direct border.
The loss of NK to SK would be a blow to China, no doubt about it. The main issue is that the US is allied so strongly to the US that their involvement in any shooting war is a flat given. But this may make China EXTREMELY uncomfortable (hell, I'm sure even SK going it alone would make them pretty upset already).
The main issue here is China's veto at the Security Council. If it comes down to a provocation so strong that SK must respond or face mass domestic unrest, then they may go to war regardless (or at least up the stakes with an Israeli-style massive retaliation). I'm pretty sure any such retaliation would need to be approved through the UN Security Council, and there's simply no guarantee that China would abstain (I'd doubt they'd actually vote for it). China's building ties with all kinds of governments in distress, so throwing one of them under the bus might still look bad. If they don't let SK respond, and SK decides it has to, regardless of whether or not it's been sanctioned by the UNSC, things could get really "interesting".
I guess it would all depend on exactly how crazy NK got and what it decides to do. Like if NK dropped a nuke on someone, obviously nobody would defend them, but that's just the extreme end of a fairly long sliding scale.
It's also worth pointing out that Japan's response is pretty important. They're quite worried about NK missile attacks and are the other large power in the region (and another US ally). If both Japan and SK start calling for an attack or full-scale war, China may begin to feel isolated and start getting funny about things.
So reaching out early - well before there's a Sarajevo 1914 moment, where one crazy idiot determines the course for all of us - is crucial. I pray SOMEBODY has the wisdom to do so.