It says on BBc site
If you're texting us on 85058, you will be charged at your standard message rate for each message.
it must be your provider that is charging the costs that way
---------- Post added at 18:06 ---------- Previous post was at 18:05 ----------
full text
Cost information
Calling 0500 909 693 is free from most landlines but some networks and mobile operators will charge you.
If you're texting us on 85058, you will be charged at your standard message rate for each message.
http://www.bbc.co.uk/5live/get-in-touch/