NI number not recognised

Hello admin I moved from Isle of Man to UK to work for NHS. I have NI number from Isle of Man which I am using in uk in 2023, July . I contribute to pension and income tax via my employer. When I tried to do self assessment for the tax, I found out my NI number is not being recognised by HMRC. I tried several times to contact 03002003300 and I was asked to wait for almost an hour and nobody picked up the phone. My employer said they have no problem with paying my income tax by using my NI number. Please give me advice as I need this tax return to be done to apply for my citizenship. Thank you Hnin

Hi Hnin Thin,
To check your national insurance number please see here:
Find a lost National Insurance number
Thank you.