You'll still need a Telephone line to get Plusnet. I feel sure it is the case that Virgin Media is the only ISP that provides Broadband without a Telephone connection; providing you property has been cabled accordingly.
Everyone (except Virgin) uses the same BT Openreach cables for the 'last few yards' to the house.
One way or another you'll be paying your service provider about £14 - £18 per month line rental for that connection. It's a total [expetive deleted] con - that network was built with our money and then given away for naff all to BT - now they charge us an ever increasing amount each month to use it. A disgusting use of a monopolistic position.
What the service providers charge for broadband/calls is where the difference comes - as stated Plusnet do do a broadband only service - but you still have to pay a line rental charge to them.
I have my internet access from BT, and very good it is as well, since I upgraded to Infinity. I don't really use the telephone line at all these days, but the whole deal worked out cheaper if the line was included. Because the vast amount of nuisance calls I get, it is "off the hook" permanently.
As others have point out, BT still provide the vast majority of telephone lines in the UK, as you would expect from a company that started out as the GPO.