The steps below are found in various CUCM documents, but step 5 needs a bit of clarification. Since the phones I was testing with are SIP based running sip88xx.10-3-1-20, you do indeed need to apply SIP dial rules to the phone devices, but configuring the rule takes one more bit of information. See my italics and pictures. This technique that includes defining the button used as the PLAR seems to be required for multiple button Cisco 7800 and 8800 series phones.
Note: if you are using Cisco 3905 phones as ring down / PLAR devices, you do not and should not define the button in the SIP dial pattern.
I've modified some of the other steps to hopefully help avoid confusion as well.
How to Configure Cisco multiple line SIP phones as PLAR / ring down phones
Create a partition, for example, P1, and a calling search space, for example CSS1, so CSS1 contains P1. (In Cisco Unified Communications Manager Administration, choose Call Routing > Class of Control > Partition or Calling Search Space.)
Create a null (blank) translation pattern, for example, TP1, in partition P1. In this null (blank) pattern, make sure that you enter the directory number for the PLAR destination in the Called Party Transformation Mask field and that the translation pattern uses a CSS that has access to that destination. (In Cisco Unified Communications Manager Administration, choose Call Routing > Translation Pattern.)
Assign the calling search space, CSS1, to either a device or line on a phone that will be dialing automatically. (In Cisco Unified Communications Manager Administration, choose Device > Phone.)
For phones that are running SIP, create a SIP dial rule. (In Cisco Unified Communications Manager Administration, choose Call Routing > Dial Rules > SIP Dial Rules. Choose 7940_7960_OTHER. Enter a name for the pattern; for example, PLAR1. Click Save; then, click Add Plar. Click Save.)
Here's the missing piece: after you click Add Plar you must define what button number on the phone has the DN that is acting as the PLAR. Leaving the PLAR default will show you a line with a blank pattern. While this seems correct as it is similar to the blank translation pattern you created earlier, it will not work.
Here's a working SIP Dial rule:
|The device where this will be applied will automatically dial when the fourth DN / button is accessed.|
In addition, if you mistakenly choose Add Pattern rather than Add PLAR and make it look just like the above rule, your PLAR will still not work.
Here's a great looking SIP Dial rule that doesn't work:
|Looks good, works badly.|
For phones that are running SIP, assign the SIP dial rule configuration that you created for PLAR to the phones (In Cisco Unified Communications Manager Administration, choose Device > Phone. Choose the SIP dial rule configuration from the SIP Dial Rules drop-down list box.)
FYI... here's another post of mine from years a ago reiterating the first several steps suitable for SCCP devices http://webmaxtor.blogspot.com/2011/05/cucm-ring-down-phone.html