I am being plagued by these just now.
Phone rings - either no-one is on the line, or you get a recorded message (something urgent I need to do NOW to avoid disaster, so press 1 NOWWWWWW!!!!!!, or send HMRC a milionty pounds to avoid an audit).
If I try to call back on the number (its usually a mobile these days) but I can't be connected for some reason (not possible or is unavailable).
Now. I get the ones that try to panic you into pressing 1 NOWWWWWW, but the silent ones that are not call-able? What's the scam here... what am I missing?