This is a UK Freephone number that originates through letter spam (regular mailbox spam in the UK) which is sent to unsuspecting residents in the UK claiming the letter is from Argos Card and stating the recipient is required to call them on this number (0800 028 2054). When one calls, scammer answers saying they owe the recipient money from 2016 and requests for the person’s date of birth and home address. Both the letter and phone number are part of this scam operation.
Argos is the digital shopping brand of a UK retailer named Sainsbury's and I have called them via the official customer cards security number (03456 400 700 from inside the UK or +44 3456 400 700 when dialled overseas) via their site (https://www.argos.co.uk/help/faq/72453) who have confirmed the number and letter are fake.
The Argos Card Security Center rep via their official hotline (03456 400 700) advised me the following: "Thanks for bringing this to our attention. I am sorry but this is a phishing letter. We are aware of it and do advise you to ignore".