I know that fax is more or less obsolete now but I came across an 0870 fax number on the Barnet website:
https://www.barnet.gov.uk/directories/council-buildings/hendon-town-hall

Do the rules for 084/087 phone numbers also apply to fax?

In our work on the issue of 084 / 087 numbers we have noted very many similar cases.

We have however disregarded them, for two reasons.

Firstly, because fax is generally used for B2B communication, which is generally treated and regulated differently. Secondly, as you say, because fax is now so fairly used, any impact is likely to be of little consequence.

[list bull-redball]

The Ofcom regulations regarding declaration of call cost apply to all telephone numbers.
 

The Government Guidance on use of Number Prefixes covers "telephone lines" and would therefore apply.
 

The Consumer Contract regulations #41 also refers to "a telephone line" and also therefore applies.
The first two of these do apply in this case.

For the reasons stated above, the fair telecoms campaign has not put any effort into campaigning for compliance in respect of fax numbers.

We are however open to hearing views, in this forum, on why this may be an important matter.