HMRC letter says I tried to pay Class 3 (voluntary) contributions - I DIDN'T!

HDUMTINC

I have received the exact letter. I checked my pension forecast on line and I also contacted the future pensions department to confirm that I can not improve my state pension as I have paid the Maximum NI. I contacted the HRMC phone number on the letter (was only on hold for a few minutes) and a very helpful  gentleman told me that they had sent the letters out on mass and just to ignore it.