You may run into scenarios where you need to configure a phone to dial a number automatically with out user intervention (i.e. elevators, emergency phones, etc.)
To create a phone or line on a Cisco CUCM system that automatically dials a number when going off-hook, there are four basic steps.
1: Create a new Partition.
This PT will be applied to a new Translation Pattern, and be referenced by a new Calling Search Space.
2: Create a new Calling Search Space.
This CSS will be applied to the device / DN serving as a ring down phone. Add the new Partition created above (step 1) to this new CSS.
3: Create a new Translation Pattern.
The pattern should be blank. The partition should be the new Partition created above (step 1). The Calling Search Space should be one that provides access to the number to be dialed automatically. The Called Party Transform Mask should be the target number to be dialed when the device goes off hook.
4. Apply the new CSS created above (step 2) to the device / DN to serve as a ring down phone.
At this point, when the device / DN goes off hook, the CSS is searched for Partitions that contain a matching pattern. The blank pattern in the new Translation Pattern is matched. Since dialing nothing is a match, the calling party, now blank, is translated to the number in the Called Party Transform Mask and that number is rung without any other input.
Update 09/28/2015: here's another how-to version including information regarding SIP phones http://webmaxtor.blogspot.com/2015/09/cucm-using-sip-phones-as-plar-or-ring.html