<P> When announced, the DTMF technology was not immediately available on all switching systems . The circuits of subscribers requesting the feature often had to be moved from older switches that supported only pulse dialing to a newer crossbar, or later an electronic switching system, requiring the assignment of a new telephone number which was billed at a higher monthly rate . Community dial office subscribers would often find the service initially unavailable as these villages were served by a single unattended exchange, often step by step, with service from a foreign exchange impractically expensive . Rural party line service was typically based on mechanical switching equipment which could not be upgraded . </P> <P> While a tone - to - pulse converter could be deployed to any existing mechanical office line using 1970s technology, its speed would be limited to pulse dialing rates . The new central office switches were backward - compatible with rotary dialing . </P> <P> In the UK, a brief excursion from standards, GPO telecoms introduced in 1968 their first push - button telephone, the GPO 726 (Ericsson N2000 series), that used neither pulse dialing nor DTMF tones . It used a DC signalling system comprising a number of rectifier diodes arranged in different polarity configuration according to the button pressed . The 726 also requires a ground wire as well as the usual A and B wires . The irony was that with the few installations, the dial signal still had to be converted to pulses in the host PABX . It is now a rare device . </P> <Table> <Tr> <Td> </Td> <Td> DTMF dialing How DTMF dialing sounds </Td> </Tr> <Tr> <Td_colspan="2"> Problems playing this file? See media help . </Td> </Tr> </Table>

When did push button phones come out in the uk