If you type Enta rather than Entanet into the search here an 01 a number comes up which was working a month back when I decided to become a customer of the Entanet reseller UKFSN. In fact they used to answer almost first or second ring.
Unfortunately they are now no longer answering this number and it just rings and rings with no reply.
So it seems like they have worked out people were bypassing the 0870 number and are now determined to force you to call it.
Only two solutions:-
(a) communicate only by email - which does seem to get answered fairly rapidly
See
https://billing.enta.net/indstart.php for addresses
or
(b) leave them as a customer and move to a supplier like
www.idnet.co.uk who use 0800 numbers for all contact.