Call Button not appearing in Microsoft Teams Client

If you have enabled a user for Teams and the call button does not appear you can check the log file why it is not appearing.

After the teams client is started press CTRL+ALT+SHIFT+1.
A small dialog appears at the bottom-right.
Click on this and the download folder opens.

Open the file named "MSTeams Diagnostics Log" + datetime.

Search for "UserAppsStore"

You will get something like this:

2018-06-18T11:29:39.132ZInf UserAppsStore: Finished adding/filtering static apps
2018-06-18T11:29:39.132Z Inf UserAppsStore: Attempting to add calling with the following flags: {"isCallingEnabledByTenant":true,"isSfbCloud":true,"isEvEnabled":null,"disableCallingForHybridVoice":true,"pstnType":"OnPrem","isBusinessVoicePath":false,"isByotEnabled":false,"isOneToOneCallingEnabled":true,"isCallingSupportedEnvironment":true,"result":"removed"}
2018-06-18T11:29:39.132Z Inf UserAppsStore: Calling app removed

In this case the the client reports that the user is not EVEnabled and it also doesn't see the OnlineVoiceRoutingPolicy being assigned.

Check your configuration or wait for replication to take place if you recently activated the user.