Have you looked at Click to Dial, this is a free widget from Cisco
Supported Desktop Applications
The following desktop applications can be used with the Click to Call application:
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Mozilla Firefox 3.5 or higher
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Microsoft Internet Explorer 7.0 and 8.0
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Microsoft Excel 2003, 2007, and 20101
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Microsoft Word 2003, 2007, and 20101
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Microsoft Outlook 2003, 2007, and 20101
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Microsoft Powerpoint 2003, 2007, and 20101
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Microsoft Sharepoint Server 2003 and 2007
According to the release notes it runs on:
Click to Call is supported on the following 32-bit and 64-bit Citrix environments:
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Citrix XenApp 4.5 on Windows Server 2003
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Citrix XenApp 5.0 on Windows Server 2008 and 2008 R2
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Citrix XenApp 5.0 Feature Pack 2 on Windows Server 2008 and 2008 R2
This is its home page
http://www.cisco.com/en/US/products/ps9829/index.html
Graham