In Outlook > Options > Add Ins, all ShoreTel add in's were showing as Inactive, with the message “Load Behavior: Not Loaded. A runtime error occurred during the loading of the COM Add-in”.
After logging with ShoreTel we were told that Hyper-V (which our RDS Servers run on) is not supported and won't be until at least version 15 so unfortunately there was not much we could do about it. However, I was introduced to someone who has a very similar setup, (Hyper-V, Office 2010, RDS with roaming profiles and same version of ShoreTel server) and their contacts import fine! So if it was working for them, there's no reason why it shouldn't for us...
I found the following site (http://www.scinaptic.com/support/0053.html) which describes how to get more information about why an add in is failing. By setting the system environment "VSTO_SUPPRESSDISPLAYALERTS" to 0, I was able to see that the add in was unable to load due to missing Office extensions for .Net 3.5 framework.
This was caused by installing Office 2010 onto the RDS server BEFORE installing .Net 3.5. (The Office extensions will only be installed if setup detects .NET is already installed)I therefore uninstalled Office 2010 and ShoreTel. I then confirmed .Net 3.5 or later was installed and re installed Office and ShoreTel.
The add ins were still inactive, but after selecting them and restarting Outlook they were now active and contacts importing successfully.