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:
•Mozilla Firefox 3.5 or higher
•Microsoft Internet Explorer 7.0 and 8.0
•Microsoft Excel 2003, 2007, and 20101
•Microsoft Word 2003, 2007, and 20101
•Microsoft Outlook 2003, 2007, and 20101
•Microsoft Powerpoint 2003, 2007, and 20101
•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:
•Citrix XenApp 4.5 on Windows Server 2003
•Citrix XenApp 5.0 on Windows Server 2008 and 2008 R2
•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