Modems and external dialers.

[Phil Budne]

See the v6 dn (IV) man page:

http://man.cat-v.org/unix-6th/4/dn

NAME
     dn - DN-11 ACU interface

DESCRIPTION
     The dn?  files are write-only.  The permissible codes are:

         0-9 dial 0-9
         :   dial *
         ;   dial #
         -   4 second delay for second dial tone
         =   end-of-number

     The entire telephone number must be presented in a single
     write system call.

     It is recommended that an end-of-number code be given even
     though not all ACU's actually require it.
....