By default, The ATA will accept a non specified amount of digits for dialing and then the user will need to use a # key to terminate the dial string. Or the ATA administrator can set up a Dial Plan:
http://www.cisco.com/univercd/cc/td/doc/product/voice/ata/ataincfg/186ugdia.htm
# ------------------------------------------------------------------------
# dial plan: alphanumeric string (199 characters max) -- dial plan rules.
# NOTE: no syntax check is performed by the actual implementation.
# It is the responsibility of the provisioner to make sure that
# the dial_plan is syntatically valid.
#
# dial_plan -- programmable strings of dial plan that allow
# one to specify:
# o optional send character to use (e.g. '#' or '*')
# o how many digits before auto send
# o send after timeout at any specified number of digits
# (time out can be changed as digits are entered).
# in the following:
# o . means match any digits
# o - means more digits can be entered, this (if needed) must
# appear at the end of the individual rule
# (i.e. e.g. 1408t5- is legal, but 1408t5-3... is illegal).
# o ># means terminating key to send is #, and termination
# can be applied only after matching hits ># (So >*
# means terminating char is *, i.e. terminating key
# must follow >)
# o rules applied in the order of listed (whichever matched
# completely first will cause trigger the send).
# o tn means timeout is n seconds (note: n is 0-9 and
# a-z -- which ranges 0 to 26).
# o more than one rules are separated by |.
# o rn means repeat last pattern n times (note: 1. ># or tn are
# modifier, they are not pattern; 2. n is 0-9 and a-z --
# which ranges 0 to 26). Use the repeat modifier to specify
# more rules in less space.
#
# You can also use the modifier 'S' to sieze the rule matching
# (i.e. if a rule matches and the modifier 'S' is seen, all other
# rules after that matching rule will not be used for matching).
#
# Examples 1:
# the set of dial plan rules:
# ".t7>#......t4-|911|1t7>#..........t1-|0t4>#.t7-"
# or equivalently
# ".t7>#r6t4-|911|1t7>#.r9t1-|0t4>#.t7-"
# consists of the following rules:
# .t7>#......t2- -- at least one digit need to be
# entered, after that, time out is 7 seconds
# before send, and terminating char # can also
# be applied after the first digit is entered,
# and after 7 digits are entered, time out
# change to 2 seconds. * means further digits
# can be entered as long as not terminated by
# timeout or #.
# 911 -- send out immediately
# 1t7>#..........t1- -- at least one digit need to be
# entered, after that, time out is 7 seconds
# before send, and terminating char # can also
# be applied after the first digit is entered,
# and after 10 digits are entered, time out
# change to 1 second. * means further digits can be
# entered as long as not terminated by timeout
# or #.
# 0t4>#.t7- -- after entering 0, if no other digit is
# entered, it will timeout and send in 4 seconds,
# otherwise, time out change to 7 seconds after
# another key is entered. again # is terminating
# digit.
# Examples 2:
# the set of dial plan rules:
# "911|1>#.r9t3.t5-|0t411t9-"
# if 911 entered, it will be sent out immediately.
# if 14088713344 is entred, after 3 seconds, it will
# be sent out but if another digit is entered (say
# 140887133445, the timeout chaned to 5 seconds).
# if 0 is entered, after 4 seconds, it will be send out.
# if 011 is entered, the time out changed to 9 seconds.
DialPlan:*St4-|#St4-|911|1>#t8.r9t2-|0>#t811.rat4-|^1t4>#.-
