I have had to contact HMRC by post about my tax refund/NI number now that I've moved back to my home country. Never heard back from them and apparently cannot contact them any other way?

Sorry, I hope this sub is appropriate for this question! I'm really not sure what to do.

I should be able to apply for the refund online, but in order to do this, I need my national insurance number, which I unfortunately I no longer know or have access to. Apparently the only possible way to retrieve this (as I can't call from New Zealand given that I no longer have a UK sim) is to write a physical letter to HMRC. I did this, providing them with all of my contact details, but I haven't heard anything back from them, and it was over two months ago now.

I have tried calling from my NZ phone and just paying whatever fee is required, but it just doesn't let me make the call.

Please could someone give me some advice regarding what next steps might be?