AI Certificate not received after submitting form CA3837

Hi there, I submitted form CA3837 on 21st September and haven't received anything yet. I have already done the work for my EU client in Luxembourg and they won't issue me with the reference number to use to invoice them until I can send them the AI Certificate. I have read other threads on this and tried the National Insurance Helpline number, but it just says it's not available and then the line drops! Please can you let me know what the current timescales are to receive the certificate (the information on the confirmation screen said 15 days) and the up to date number to call to chase this up?

Hi Frederika Roberts,

Sorry for the problems that you’re experiencing getting through to the National Insurance Helpline.  If you could get through to them they will be able to ensure that they have received your request and give you a timescale as to when you might receive a reply.  The telephone number for the National Insurance Helpline is 0300 200 3500.
I can, however, advise that the Team that deal with your query are currently dealing with queries received in July 2023 so unfortunately there will be a delay before they reply to you.

Thank you.