On Windows 7 if you clicked a url in the format tel:447775560821 then (if Lync 2010 was installed) you would get the following:
and you would be able to click to call from the webpage/Outlook signature/et al.
In Windows 8 the
and doesn't let you choose Lync 2010 as an option for Tel (the full href list is on Tom Kisner's blog).
This only happens if you have another program which takes hold of the URL:tel protocol as a fresh install of Windows 8 and then only installing Lync 2010 (no Skype or Chrome installed) allows Lync to get the call.
More mystifying is that if you look in the registry then the URL:tel file association is:
and I have been unable to find where Chrome is taking over the association.
Strangely by clicking the "Look for an app in the Store" option that Lync Modern UI is not listed as an option:
As we are not moving to Office 2013/Lync 2013 at the moment I've not had a chance to test if the newer version fixes this.
There is a fix for this which mentions Lync 2013:
For Lync 2010 the Reg key is at: