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The community's VoIP FAQ contains this post here:
Jan 25, 2016 Question:How many Lines can I register using Polycom phones with Lync Server / Skype for Business server ?
Resolution: You can register only one line when Lync is set as the base profile on the phone. The RealPresence Trio has Hybrid registration Feature from UC Software 5.5.2 or later. Details can be found in the Trio Admin Guide.
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