Changing the Phone Extension in Avaya Communicator for Microsoft Lync 2013 after the initial configuration

Problem

You’ve completed deploying the Avaya Communicator for Microsoft Lync (version 6.4.0.2.50) client side integration with Skype for Business Server 2015 to replace the previous Lync Server 2013 and Avaya AES server’s RCC functionality but noticed that you are unable to change the Extension in the Avaya Communication for Microsoft Lync because it is greyed out once it has been configured:

Solution

I’m not exactly sure if this is by-design but one of the ways I found to workaround this was change the Choose the Device for Calls: setting to another option (in this case it is the Computer setting):

Then change it back to the previous Desk Phone option and you will then notice that the Extension field is no longer greyed out: