If you have PSTN conferencing enabled, then people can dial into a Lync-hosted bridge using anything (PSTN, Skype, cell phone etc.) That will bridge the audio portion of the meeting. You could then use GoTo Meeting for the video portion.
If you are asking if you can use any other software client to join a Lync meeting for visuals, then no. It must be Lync (or a 3rd party app with native UCWA support).
Have you visited Lync News lately? All of the latest Lync news, articles, and tips collected in one giant aggregator. http://lyncne.ws
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