Consider the following scenario:
You add a new SIP domain in a Microsoft Skype for Business Server 2015 environment.
You change the default SIP domain to the newly added domain.
A user signs in Skype for Business mobile application by entering the telephone number.
The user exits the Skype for Business mobile application and then signs back in.
In this scenario, the user can't sign in and receives the following error message:
Cannot sign you in... try later.
This issue occurs because the Dial Plan contains non-numeric or non-alphabetical characters. This causes the telephone number validation failure.
To fix this issue, install the June 2016 cumulative update 6.0.9319.259 for Skype for Business Server 2015, Web Components Server.