As timl says. It is the broadband part of 21CN. Initially it provided ADSL2+ from BT Wholesale. Nowadays all the FTTC stuff goes through WBC exchanges as well, though not necessarily the same exchange as your copper line.
A lot of Plusnet high cost exchanges were still 20CN, but BT Wholesale are upgrading those. 21CN wholesale prices are much lower than 20CN. Though Plusnet did continue to charge their higher price even when the upgrade happened
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