Consider the following scenario:
You are using Microsoft Lync Phone Edition for the Aastra 6721ip or Aastra 6725ip telephone device.
You configure multiple emergency numbers, with or without assigned dial masks.
The number of dial masks that are assigned to any one emergency number is less than the total number of emergency numbers that are configured.
In this scenario, when you sign in to the device, the device crashes.
This problem occurs because the device can’t correctly parse each emergency number and its corresponding emergency dial mask.
To resolve this issue, install the April 2017 cumulative update for Lync Phone Edition for Aastra 6721ip and Aastra 6725ip telephones.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.