Option 1:
UCS 5.0.2 or later changed the behaviour in displaying the registration name rather than the Extension Number.
UCS 5.0.1 or earlier:

UCS 5.0.2 or later:

In order to change this back to the prior behavior please use:
<nameURI reg.1.useTelUriAsLineLabel="1" />
Option 2
Changing the Line Label manually to a desired name.
LYNC usually utilizes the User Logon / SIP URI to populate the Line Label on the Phone.
The above result may not be desirable
Original:

Change via the Web Interface:

or Configuration file:
reg.1.label="Steffen Baier"
Result:

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The title Polycom Employee & Community Manager is a community setting and does not reflect my role. I am just a simple volunteer in the community like everybody else. My official "day" Job is 3rd Level support at Poly but I am unable to provide official support via the community.
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Notice: This community forum is not an official Poly support resource, thus responses from Poly employees, partners, and customers alike are best-effort in attempts to share learned knowledge. If you need immediate and/or official assistance please open a service ticket through your proper support channels.
Please also ensure you always check the VoIP , Video Endpoint , Skype for Business , PSTN or RPM FAQ's