An email alert from the DSA says that from 2 April 2013, phone lines for booking tests will only be 
available between 8am and 12pm (instead of the current 8am until 4pm). This is in line with the falling numbers of people booking via telephone.
Booking online is by far the easiest way. You can book your theory test and practical test on the DSA’s site at GOV.UK.
There are still a bunch of radical ADIs who oppose this on the grounds that their “older” pupils might not have access to, or be familiar with, the internet. Quite frankly, those people are likely to have issues with any booking system.
We’ve come a long way since Victorian times. It’s a shame that some people still want to stay firmly embedded in the past, though. The majority of people can easily book via the internet. Those who can’t will be able to use their ADIs to book it for them. There isn’t a problem unless you look for one.