Why does it say check date of birth when I enter mine

[Liverpool25]

I am trying to change mine number on my new 3 account to my exciting number from tesco but when I enter my date of birth it says my date of birth is right 

[sc1999]

Hi, it's a known problem and 3 are taking their own sweet time in fixing it. Go to this page and use one of the contact numbers or chat and a human should sort it out.

https://www.three.co.uk/support/contact-us