Spark needs a STUN server to be able to create a peer to peer media connection with another Spark client. STUN server helps them find each other. In previous versions there was a STUN server "stun.xten.net" hardcoded into a Spark. This is obviously not an option for a corporate environment, when a client is making a connection to a third party server. So it was removed and so a Call (Computer to Computer) button disappeared from Spark menu (it appears dynamicly if it can find a STUN server).
To enable this in 2.6.0 version of Spark, enable STUN service on your Openfire server. In Admin Console go to Server > Media Services > STUN Settings and check Local Server Enabled. Then select Primary and Secondary IP addresses. To be able to do that, your server has to have 2 IP addresses and also each would be assigned with a different port (and those ports should be opened in a firewall). Also you can delete all the External servers if you want to be safe, or you can add external server, which you think you can trust. Notice, you still need to check Local Server Enabled if you want to use external servers only.
Then you should go to Spark profile and edit spark.properties file and add anywhere:
You will have to modify properties files of at least 2 Spark client to be able to see Call button when you open chat window for these two contacts.
In the current Spark versions there is a field to add Stun Fallback server in the Preferences > Media Settings