This is a bit of history...
When BT were the only telco in the UK, one of the things they had was a light user scheme. This was a totally automatic system which meant that if you used below a certain number of call "units" in a month you got an automatic discount per unused unit off your line rental.
It meant those not making any calls, or few calls, got line rental at a much lower level. It was the "social" tariff of the day as there were no "call bundles" or crap like that.
I remember those days, BT was simple to deal with, and they had sensible compensation for faults. Why did we ever change? Oh well...
I had a phone line, and back then I was living seriously on a budget. I also had a BT calling card allowing you to enter a long code on a phone box to make a call on your home phone bill. That itself was fun, as the payphone sent a DTMF sequence reporting its number and you could hear it do that. I did decode that once using calling party clear calls to the payphone.
The fun thing was that calls from my home phone line were circa 5p per unit, and calls using BT calling card were circa 10p per unit, both on my home phone bill.
I used Mercury when it was launched. It meant that most actual calls were made with a prefix and a code and charged by Mercury and not BT. So my BT usage was "light". As such I benefited from the light user scheme.
Bit I did, occasionally, use my BT calling card. And those calls appeared on my bill. As this meant some usage then the light user scheme rebate was impacted by my calling card usage.
However, the calling card had the impact that each 5p I spent reduced the rebate as one unit, and not each "unit" I spent (remember the calling card charges 10p/unit not 5p/unit).
I complained, and was told the advert/leaflet was not the "formal" price list, and that explained the actual rules. So I asked for the price list.
I got a thicker glossy brochure that said the same, the rebate is unused units not unused money spent. But it too had the caveat of not being the formal price list.
Eventually after many calls to BT they put me on the mailing list for the actual price list, and it was a big blue binder (later to be two binders) with every single service BT sold, and the pricing. From fractions of a penny for some services to millions of pounds for others. It was really interesting for a geek like me. It was a really useful reference many times when dealing with BT for many years.
It said the same, unused "units" get a rebate, not unused "5p spent". I won my argument over like 25p of line rental.
They eventually changed the rules not to allow use of Mercury for the social tariff, which is a lot like what we see now, charging more if you dare use someone else for your calls. Now it is charging more if you dare use someone else for your broadband, and we (AAISP) are on the receiving end of that bad contract being allowed under UK telecoms regulation. I see the need for a social tariff, but not the need for a price that depends what else you but from competitors.
But there is a funny story, such that I should thank BT for their ineptitude, hugely. I would not be here today if not for the above story. I'd be working for some large telecoms company still, I bet.
One day the courier that delivered the frequent (every week to two) updates to the formal BT price list (yes, on paper), accidentally mixed up the two distinctive Tyvek envelopes and delivery my update to someone else in Bracknell, and his update to me. Or may be the other way around. But the courier wanted to fix it when he realised, and the result is I ended up in touch either the other telecoms pedant in Bracknell, now a friend of mine, Kev. We went on to make FireBrick after many meetings in a pub, and that is the basis of our Internet service at AAISP.
So thank you BT for being so inept that I signed up for the BT Price List.