How bizarre. It rather seems to contradict :
"To start enjoying the Essential package you will need to switch to a Telewest phone service. Our Talk Weekends phone package costs just £11.00 a month, including line rental and unlimited local and national weekend calls (Mobile, international, internet and non-geographic calls (like 0845 and 0870) and premium rate numbers are not included. These are charged at our standard rates.) Alternatively you can choose either our Talk Evenings & Weekends Talk Unlimited packages."
on
https://www.telewest.co.uk/websales/product.do?id=20
and similar statements dotted all over the website
If I have a spare few hours to spend listening to assorted music I'll give Customer Services a ring and ask.