Voluntary NI 18 digit number - do I have to phone to get it if I have all info on-line?

I have checked my online HMRC account for my State Pension Forecast and NI contributions which clearly say to get the max state pension I need to make 1 more year NI contribution and I can make voluntary contribution payment for 22/23 tax year as no NI was paid in that year. It even tells me the amount I would need to pay is £824.20. However, in order for me to make the payment I need the 18 digit Class 3 NI ref number, but to get this as far as I can see I have to ring the future pensions phone number 088 7310181. This line is so busy and I cannot get through. It doesn't make any sense to have all the info on line in my account and then require a phone call to get the ref number. Presumably if I ever get through on the phone they will just tell me the same info and give me the number. Is there any other way of getting this 18 digit number other than by phoning ?

Hi anne.ronchetti,
To get an 18 digit reference for Class 3 voluntary contributions payment you should call our NI helpline (not DWP) 0300 200 3500 or there is an option get get an 18 digit reference via a webchat operator on Contact HMRC
Thank you.