While this enabled interactive services and a crude form of e-mail to be provided, gaining access to Prestel also involved purchasing a suitable terminal, and arranging with a Post Office engineer for the installation of a connection point known as a Jack 96A. (From the early 1980s, the "New Plan" sockets were fitted as standard on new lines and on any change of rented handset, and terminals or modems then required no special connections.) Thereafter it was necessary to pay both a monthly subscription and the cost of local telephone calls. On top of this, some services (notably parts of Micronet 800) sold content on a paid-for basis. Each Prestel screen carried a price in pence in the top right-hand corner. Single screens could cost up to 99p. The original idea was to persuade consumers to buy a modified television set with an inbuilt modem and a keypad remote control in order to access the service, but no more than a handful of models were ever marketed and they were prohibitively expensive. Eventually set-top boxes were made available, and some organisations made these available as part of their subscription, for example branded Tandata terminals were provided by the Nottingham Building Society for its customers, who could make financial transactions via Prestel. Because the communication over telephone lines did not use any kind of error correction protocol, it was prone to interference from line noise which would result in garbled text. This was particularly problematic with early home modems which used acoustic couplers, because most home phones were hard-wired to the wall at that time. Regardless of the hardware choice Prestel was an expensive proposition, and as a result, Prestel only ever gained a limited market penetration among private consumers, achieving a total of just 90,000 subscribers, with the largest user groups being Micronet 800 with 20,000 users and Prestel Travel with 6,500 subscribers. The costs for businesses interested in publishing on Prestel were also expensive. This ensured that only the largest or most forward thinking companies were interested in the service. During the daytime, when business usage was high, there was a per-minute charge to use Prestel, but in the evenings and weekends, traditionally the quiet times, it was free apart from the telephone call. With Micronet being so popular, suddenly the quiet times became fairly busy. The BT Prestel software development team developed a number of national variants of