There's a second reason why PN might have put a cease on your line, without your account ending. That's the possibility that PN are moving your from their old BT-PN interconnect, onto their new BT-PN interconnect.
This is because PN are shifting from a "Shared WBMC" network to a "Dedicated WMBC" network, which affects the way that BT wholesale pipe the data between Plusnet and your exchange. unfortunately, that shift is best done "under the hood" with a cease-and-reprovide ... which isn't fully invisible.
If this is the case, then you might find some odd "closed tickets" on your account...