If you are using the VIA Voice Skype for Business Cloud on a trial/pilot and you cannot change your DNS, you may sign into the Desktop Skype for Business Client using a "Manual Sign in Address".
In the Skype for Business client, click the cog or gear towards the top of the client:
Select, "Tools" then "Options", "Personal" then "Advanced". You should then see the following screen:
Enter the following into the "External Server name" field:
Ensure this is ONLY entered in the "External server name".