I've used various over the years but am now with NTL and cannot recommend them enough ....
NTL's £15/month 0800 unlimitted is fantastic, plus you get £15 worth of calls free at a good rate (3p peak, 2p off peak, 1p w'ends).
Way better value than BT or Freeserve deals.
I get a connection every time, never had an engaged tone, never had it drop an in use connection .....
I've used other 0800 deals before but they have all had gripes, (BTW I dont work for NTL !)
Happy surfing !