GhostRiderUK

Hi, I am a high rate tax payer. Through ignorance, I did not know we need to individually claim back the tax relief (higher rate tax payer) on pension contributions. I managed to do this successfully for the 2023 tax year on my self assessment, that I filed in January. But I didn't see a way to do this for the previous 3 tax years, which I believe I can still do. Is the only way I am able to do this, via phoning the 0300 200 3300 number? I have called them 10 times today, I get a message saying the average wait time is 40 minutes (based on yesterdays statistics) however 10 seconds later, it says that it's too busy to take my call and it immediately terminates the call. I have no option to hold, even though I want to. How do I get in touch, if it won't even let me hold? I do not believe I can claim for previous tax years online, can I? I'd rather not write in via the post... any solutions?