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.
....
