In 1980, NTT began selling a "vibrating silver phone" telephone unit based on a bone transmission system for use by hearing impaired customers. This same technology has been applied to develop stereo headphones which can be used by the hearing impaired at rock and pop concerts. In 1993 a new version of this product, called the improved live phone "Tokimeki", was finished for additional application at classical music concerts and movie theaters. This product allows hearing impaired individuals to enjoy musical concerts held across Japan. In order to use the live phone "Tokimeki", it is necessary for each individual to be fitted for volume level, sound range and balance according to their particular level of hearing ability. Although this personalized fitting was originally performed by specially trained technicians, improvements to the product's design allow this to be performed by volunteers made up of NTT employees working together with sign language interpreters.