We need to focus on something far more important.
If I'm phoning the helpline... you've already failed. It means the tax system isn't clear and obvious, it means that answers cannot be simply found online, and it means that online channels have proved insufficient.
If this happens at precisely one time of the year in excess, too, it means something else is broken: Making everyone file on the same day. It's a nonsense. You could stagger them throughout the year and instantly spread that load across 365 days instead of a handful near tax return time (it's not like that payment is going to happen instantly, it's not like everyone is billed on the same day, etc.).
Someone having to phone HMRC at all means... you failed in the 21st century.
Personally, as a mathematician who was self-employed for nearly 10 years, I avoided everything I could possibly do to make my tax return as simple as possible, precisely because without a qualified and experienced accountant it's impossible to accurately determine whether something is allowed or not, whether there's something else you could claim for or not, where the boundaries lie on any definition, or even what the case law says about exactly what things apply to. I literally took to just putting down my income in one box, filling out all the related calculation boxes (this was when they were still trialling the online tax return), and put zero or N/A in everything else. I could have claimed for ALL KINDS of things back, but it honestly was not worth the time and effort to do so, especially if you got it wrong about something being allowed. The numbers didn't scare me. Even filing it online wouldn't have scared me. But simple things like "what can I claim back and what can't I and how long do I have to keep the evidence of it all" just meant that it wasn't worth my time or hiring an accountant to do it for the odd travelcard and an occasional spot of lunch. Literally, income = X. Expenditure = NIL. Anything else wasn't worth my time to determine it to their extraordinarily un-disclosed criteria to either of our satisfaction.
If you made the tax system simpler, if you tied tax return dates to, say, company start dates, and if you actually had the information and assistance available online... you would probably never need the phone line at all.
And when it comes to benefits, it's quite simple. Tell people what they're entitled to based on HMRC's knowledge, write them a letter (or an online notification) every month reminding them of any eligibilty or circumstance change, and then police that effectively so that people can just simply tell you "Oh, yes, someone lives with me now" or "I got divorced" or whatever, and when it appears on their dashboard every month they can't deny "not knowing". And you can't deny that it would be far simpler to just allocate the benefits according to that data than faff around expecting elderly/disable/etc. people to magically know what they might be entitled to.