Sometimes happens that you receive a Lync meeting request, but when you click on the meeting URL the Lync client cannot join the external SIP domain. In these cases the simplest solution would be to launch the Lync Web App, however it won’t start in the browser if you have the Lync client installed.
The quick solution is to append ?sl=1 to the end of the meeting URL.
For example: https://FQDN/organizerID/meetingID?sl=1
You can read more about the internal launch logic of the Lync Web App: Launching Lync Web App.
I am not sure that this is a best practice to follow in 2013.