Assume that you disable the Call Via Work feature in mobility policy and set your work phone number and mobile phone number to the same number in a Microsoft Skype for Business Server 2015 environment. In this situation, you can't set the same mobile phone number in Skype for Business mobile app.
This issue occurs because the Unified Communications Web API (UCWA) blocks setting of mobile phone number when it's the same as the work phone number.
Article ID: 3209570 - Last Review: 2017 ots. 13 - Revision: 2
Skype for Business Server 2015