SIP dial rules are essentially a local dial plan configured on a SIP phone that allows the phone to collect all dialed digits and act on them before being sent to the CUCM.
Furthermore, not all SIP phones support KPML (keypad markup language). KPML allows the digits to be sent as they are pressed to a CUCM, and subsequently be analyzed by the dial plan on the CUCM. However, doing so will cause additional processing on the CUCM and subsequently reduces the performance of the call processing agent on the CUCM cluster. As a result, Cisco recommends configuring SIP dial rules to be configured on SIP phones to offload this performance hit.
Note that SRST does not support KPML, but will instead continue to use the previously configured SIP dial rules configured by the CUCM.
Phones that have KPML: 7911, 7941, 7961, 7970, and 7971
Phones that DO NOT have KPML: 7905, 7912, 7940, and 7960
For phones that do not have KPML, if SIP dial rules are not configured, the user must press "Dial" or wait a specified amount of time before the dialed digits are sent to the CUCM.
Also be aware that the 7905 and 7912 phones have a different set of operators used when the dial patterns are defined.
Extension mobility supports 7941, 7961, 7970, and 7971 (SIP).
More Details hereand here.