Enterprise Alert supports parallel voice calls for example for group calls or conference bridges. In Enterprise Alert 2016 there is no longer a license restriction on the number of voice lines. That means you can use as many lines for parallel voice calls as allowed by hardware and network.
However, there are still the settings for “Maximum Inbound & Outbound Lines” and “Reserved Inbound Lines” in the “Voip” and “Lync” / “Skype for Business” modules.
You might want to adapt these settings to match the number of lines you would actually want to use in your voice-call scenarios.