Drop Dial Premium Rate competitions haven't taken off in such a big way online as many first thought. Essentially they are premium rate competitions, but online. In order to enter, you required to connect with an expensive phone line. Prizes tend to be of high value, but the cost to enter is £1 and upwards. Payment can sometimes also be made using BT click&buy.
With the web dialler option, the connection to your ISP is dropped, then reconnected via a premium rate line in order to collect your entry fee. Once this is done, you are then reconnected back to your ISP.
Note, you should receive warning that you're about to enter a drop dial competition and there should be clear instructions on the website, including the cost and length of calls.
Our advice for these kind of competitions is be careful, and only enter via websites with well known and reputable brand names, e.g GMTV, This Morning.
If you're concerned about drop diallers, have one on your PC that you need to remove or want some more information then Web User have a mini guide that may be useful.