From the TechNet here are the Skype4B pre reqs links you need

• Windows Firewall Service must be running

Disk Space

• At least 32Gb of available disk space !

Latest Lync 2013 Server CU (Feb 2015 currently)

PowerShell RTM version 6.2.9200 or later

SQL 2012 Express requires at least SP1 but SP2 is available

• SQL 2012 Express SP1
• SQL 2012 Express SP2

OS Updates

I would really think if your using 2008 R2 OS life time and improvement to Windows Fabric requires Server 2012.

Recommend OS for Skype4B Server 2015 is 2012 R2.

• Server 2008 R2 – KB2533623
• Server 2012 KB2858668
• Server 2012 R2 KB2982006

Upgrade Workstation

To perform an InPlace upgrade you will need a workstation / Server they has no Lync admin / tools installed 🙂