Opened a TAC case regarding this issue - CUCM 10.5 / 8851 phone in my case.
Here's TAC Response
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I have checked into this and this is working as designed.
++ Phone's GUI will display the External Phone number mask of Primary Line only And Phone will display only the directory numbers for other lines apart from the Primary line.
This is working as per design.
++ Currently there is no option in getting the EPNM (External Phone Number Mask) displayed for the lines other than primary line. Only primary line which is the FIXED line for the phone (which cannot be modified to any other options) will show the EPNM in the phone's GUI.
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I asked where this was documented:
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I don’t have any document to share about the same. I checked this internally and other support cases as well on the same. The developer side has mentioned that currently this is working as designed. Only primary line which is the FIXED line for the phone (which cannot be modified to any other options) will show the EPNM
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