The ISP has put out a second, reactive ad with the same image but a new tagline, after customers pointed out the error on Twitter and via email.
The new ad reads: "Yesterday we ran this ad saying we were "blowing our own trumpet". We got it wrong. The instrument depicted in the ad IS NOT a trumpet, it’s a cornet.
"Clearly distinguished by its conical bore and compact shape. Thanks to Jean Mottershead, of the internet, for pointing this out."
The original print ad ran on Monday this week, with a number of consumers spotting the mistake.
Oi, Plusnet: that's a *cornet* pic.twitter.com/JJgCMjA2Tn— Dave Rutt (@rutty_uk) April 20, 2015
A Plusnet spokesman told Marketing that brass instrument pedant Jean Mottershead is a real person.