The best replacement for enterprise voice is remote call control.

The best alternatives to enterprise voice are rated as following:
★★★★★  remote call control

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Speaking about enterprise voice, one probably shouldn't forget about lync online.

Other search terms related to enterprise voice and their ratings are:
★★★★★ lync online
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Google Trends

Scale is based on the average worldwide traffic of enterprise voice in all years. Click on graph for more.
enterprise voice
remote call control