Looks like it's happened to me now.
A 2 second call, costing 45p, to 123. No-one at home that day.
I'm on BT, not Plusnet, but I don't think that matters - I am guessing it was a test from the roadside cabinet - e.g. perhaps to find a free line for a new customer. I didn't have an open fault at the time.
I've complained to BT and they say the charge will be removed from the next bill - plus the feedback/complaint will be shared with Openreach.
BT ADSL customer getting 3.8 Mbps (0.7 Mbps up) on a new road / new build development
(It was around 1.6 to 1.9 Mbps when I moved in)
CAB not FTTC enabled, not part of the 66% commercial plan. Not a BDUK area. Hoping the council will let VM in-fill.