PLAR is a common feature often used for elevator call boxes, emergency phones in public areas, or even video enabled door phones.  The thought is that when someone takes the phone off hook a call is immediately placed to someone who can help.  It’s a critical configuration to get right and test before giving any stamp of approval on system configuration.   The basic configuration idea is that an empty translation pattern must be created and accessible to the PLAR phones from a Class of Control standpoint.  This translation pattern will then re write the empty called digits to the digits of the destination phone that should be called.  The functional differences of the SIP and Skinny (SCCP) call control protocols make the process a little different for each phone type.  SCCP phones go off hook and send digits one by one.  Each time a new digit is sent to Unified Communications Manager all of the digits are compared to the Directory Numbers, Route Patterns and Translation Patterns the phone’s Calling Search Space has access to.  When the SCCP phone goes off hook and no digits have been sent a blank Translation Pattern will in fact match and start a call.  A SIP phone, however, does not invoke any matching until dialing begins meaning we have to take a slightly different approach.

1. Create a Partition in which the translation pattern can be created. You must have a Partition that’s essentially exclusively reserved for this translation pattern.  I suggest naming it so that it’s easily recognized.  Partitions can be added under “Call Routing -> Class of Service -> Partitions”.
2. Create a Translation Pattern inside the Partition created above. Do this at “Call Routing ->Translation Patterns”.  This Translation Pattern should match a blank pattern in its “Translation Pattern” field.  Set the Translation Pattern’s Calling Search Space to a Calling Search Space with access to the destination number you would like to have the PLAR call placed to.
3. Create a Calling Search Space that can access only the Partition created in step 1. This can be built under “Call Routing -> Class of Control -> Calling Search Space”.
4. Assign the Calling Search Space to the phone that should be doing the PLAR calling. Remember, the line Calling Search Space takes precedence over the Phone Calling Search so configure accordingly.  If you configure the Phone ensure the line Calling Search is set to <none>.

If you are configuring a Skinny (SCCP) phone you should be all set.  A SIP phone, however, takes a few additional steps.

1. Create a SIP Dial Rule by navigating to “Call Routing -> Dial Rules -> SIP Dial Rules”. Chose “Add New” and then select the “7940_7960_Other” option from the drop down.
2. You will have the option to name and describe the rule. I suggest doing so.  Hit save.
3. The next page will have a “Pattern Addition” section with a “Pattern Description” field. Fill in another meaningful description.  Of the two buttons “Add Pattern” and “Add Plar” you must chose the PLAR option otherwise this will not work.
4. The next screen offers a “Dial Parameter” dropdown and “Value” field. Select “Button” in the drop down. The “Value” should be the line number on the particular phone that should have the PLAR functionality added to it.
5. Locate the phone you are configuring and adjust the “SIP Dial Rules” field to the name of the Dial Rule you just created.
6. Save and Reset the phone configuration.

Essentially, what this dial rule is doing is sending a blank number pattern to Unified Communications Manger when the phone goes off hook.  When this empty pattern is processed it drops into the same Translation Pattern our SCCP phone used making PLAR work in the same way.  I have used this configuration on a number of devices including SCCP registered analog ports on voice gateways such as the VG350, 7900, 7800 and 8800 series phones, and even the DX80.  The only devices that do not seem to support this are telepresense video endpoints as they lack the SIP Dial Rule option field.

I hope you found this helpful!  Please feel free to comment below.