Well they wouldn't get the service from BTOpenreach if they didn't or wouldn't want to pay for what they charge.
Of course they won't be paying BT more than they charge you - they would be out of business of that was common practice.Stands to reason Zen would advise of reduced cover and advise the new provider does it - Spent years at an ISP having my ear bent over phone faults with other providers, people thought because their ADSL was with that ISP then it was also our job to sort the rest out.
BT recently enhanced the fix time anyway - but also after the 12 hours Zen can and I am sure do claim the £35 per 24 hour compensation that BTO pay.
Sorry, you're still talking at cross purposes.
ZEN are saying they provide 24/7 Technical Support. Not 24/7 fix. They have said that if it takes them longer than 12 hours to fix a line they'll give you a £25 credit.
This is NOT the same as them saying you can have it fixed on a 24/7 basis. For example, Plusnet offers Business customers 24/7 technical support, but it is not all inclusive and does not include line fixes - and even doesn't include fixing all types of issues that might be reported.
It is also not as simple as you make out that Zen must be paying BT for this service - there's absolutely nothing to say they do that I could see, as there are 4 care levels for example on the PSTN, it could be on ANY of those 4 levels. If the customer chooses not to use Zen for the line rental, unless that customer has had the line provider (eg the company paying Openreach for the WLR3 service), provide a suitable enhanced care a fault that takes out the PSTN line will NOT be something Zen could get fixed.
Equally, as there is both an enhanced care available for the WLR3 (eg the copper pair, PSTN line) *AND* another enhanced care for the broadband part (that Zen may or may not be able to use since it will sometimes not use the BT Wholesale service), a further complexity and difference may exist.
It is entirely possible that Zen might choose (as could any other provider) to offer a service that includes a compensatory element (eg the £25 credit) if they can't get you sorted in 12 hours but without needing or choosing to pay for enhanced care levels upstream. For the simple reason, it is likely commercially cheaper for them NOT to do so. Whether they do so or not in practice is a different matter, and which care level they do or do not use for each component is not clear.
For example, care level 3 offers clear by end of same day if reported by 12:59, or 12:29 the next day if reported after that.
Care level 2 offers response within 4 working hours (not all hours!), and clear by end of next working day.
Critical Care offers a 6 hour fix on a 24/7/365 basis.
...but Openreach exclude all manner of events - and it is not clear from the Zen stuff I've read what they do or do not include - for example if a major flood takes out service, Openreach will declare an MBORC and will not compensate... does Zen??
There are many questions not answered and I merely am trying to answer the OPs question but suggest a level of caution in taking the words as printed.