I did think at first that silence doesn't give more away, much as it may make things harder for Narcissa. And Narcissa's skilled at dissembling, even with no warning, but he does know her very well.

A note, my first thought was an anonymous owl from somewhere to Cottesmore - his London house - with instructions for his elf to bring it to him directly. I'm not sure it'd work if Barty were at home, but we could sort out the timing so it'd be unlikely.

Or, I suppose, there's coded messages in marzipan, sent to the school. I sent him something of the kind a couple of years ago, charmed words onto marzipan books, and I don't think it'd put the source at risk, just make it clear I had a way to get my hands on some. (They're in New London, and again, easy enough to lay hands on.)