I'd recommend you to read this paragraph
Dialed Pattern Recognition
Digit strings dialed by a user on a telephone generally follow patterns. For instance, many enterprises implement a five-digit abbreviated dialing pattern for calls made within the same office location. Also, many enterprises rely on a single-digit access code to represent outside dialing, followed by some quantity of digits to reach a local PSTN number or a long-distance PSTN number (for example, 9 followed by seven digits to reach a local number, or 9 followed by 1 and ten digits to reach a long-distance destination).
The system administrator must plan the system's recognition of these patterns to ensure that the system will act promptly upon detection of a string that corresponds to a predetermined pattern so that users experience no (or minimal) post-dialing delay.
For phones using the Skinny Client Control Protocol (SCCP) and for SIP phones using the Key Press Markup Language (KPML) during dialing, you can implement pattern recognition in Cisco Unified Communications Manager (Unified CM) by configuring route patterns, translation patterns, phone DNs, and so forth. With each digit dialed by the user, the phone sends a signaling message to Unified CM, which performs the incremental work of recognizing a matching pattern. As each key press from the user input is collected, Unified CM's digit analysis provides appropriate user feedback, such as:
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Playing dial tone when the phone first goes off-hook
•Stopping dial tone once a digit has been dialed
•Providing secondary dial tone if an appropriate sequence of digits has been dialed, such as when the off-net access code 9 is dialed
Once digit dialing is completed, Unified CM provides user feedback in the form of call progress tones, such as ringback tone if the destination is in the alerting stage or reorder tone if the destination is invalid.
IP phones running the Session Initiation Protocol (SIP) can be configured with pattern recognition instructions called SIP dial rules. When used, they accomplish the bulk of the task of pattern recognition within the phone. Once a pattern is recognized, the SIP phone sends an invitation to Unified CM to place a call to the number corresponding to the user's input. That action, called a SIP INVITE, is subjected to the Unified CM dial plan in the same way a call from an IP phone running the SCCP protocol would be, except that Unified CM's digit analysis is presented with a completed dial string (that is, all of the digits entered by the user are presented as a block to Unified CM for processing). In this mode of operation, user feedback during the dialing of the digit string is limited to what the phone can provide (see SIP Dial Rules). Once the string has been composed, user feedback can still be provided by Unified CM in the form of call progress tones.
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/srnd/8x/dialplan.html
HTH
java
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