At that time, the intent for area code 900 was as a choke exchange—a code that blocked large numbers of simultaneous callers from jamming up the long distance network.
Numbers with the 900 area code were those which were expected to have a huge number of potential callers, and the 900 area code was screened at the local level to allow only a certain number of the callers in each area to access the nationwide long distance network for reaching the destination number.
These are theories, of course, since I am an actual adult.
Earlier, 976 numbers used 976 as a local prefix (970 or 540 in some markets like New York state), though it was not assigned to a specific telephone exchange like other prefixes.
In many European countries, for example France, Germany and the United Kingdom, it was common for organisations to operate customer service lines on premium-rate numbers using prefixes that fall outside the scope of the country's premium-rate number regulations.
Therefore, in contrast to North America where customer service numbers are typically free of charge to the caller, consumers in Europe often used to pay a premium above the cost of a normal telephone call.
Also, the early incarnation of 900 was not billed at premium-rate charges, but rather at regular long distance charges based on the time of day and day of week that the call was placed.
The number used for the radio program was one that was specially arranged by AT&T Corporation, CBS Radio, and the White House, to be free to the calling party.