The EMI Voice Letter was a 3-inch reel of ¼-inch magnetic recording tape that could be recorded and played back on a standard open reel tape recorder. Once recorded, the reel would be placed back in its packaging which had space on it to write an address and to attach postage stamps to send it to someone. The tape could record 10 minutes when run at 3¾-inches per second.
Similar reels marketed for keeping in touch by posting voice recordings were available around the same time, such as the Mastertape Voice Letter and the Scotch One Five Special. The Smith Corona Mail Call Letterpack of the late 1960s was a similar concept. Postage overseas was much cheaper than making an international telephone call at the time.
The EMI Voice Letter does not seem to have become very popular, and would probably have been superseded by smaller and more convenient products like the Compact Cassette.