Hi, where would I input or find "ims_volte" or did you mean this app https://play.google.com/store/apps/details?id=com.zacharee1.systemuituner
I've been using their 4GB All-in-One PAYG offering for years, currently £11 for 28 days, and have just switched to the 4GB pay-monthly plan for 2 years. It comes out to the same amount when paid via Direct Debit.
Three will probably argue that they're offering a discount for paying via DD and using the marketing anchoring trick of inventing a higher price they don't expect anyone to pay and then "discounting" it. It's common practice, especially with power and gas companies, landline and mobile carriers, ISPs, etc.
Amazon is doing this today: I just picked a random item from their front page and it has "RRP £149.99. Price £118." A £32 "discount"!
Presumably it depends on whether anyone pays monthly by a non-recurring method, and whether the equivalent PAYG pricing can be used to demonstrate it. I'd be interested to see what, if anything, comes of your complaint.
I suspect they'll say the guidance you refer to is meant to apply to businesses that add a surcharge to the listed price (e.g. in infamous 5% charge if paying by credit card).