Maybe a simple 'popup blocker' issue? 

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It works for me just fine on Windows 10.  If you double click the icon in the taskbar does it show up?  Curious if you somehow have the Run Application minimized in Settings, General.

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This has happened to me before. Try killing the app through task manager, and then open it again. 

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I am running Windows 10 Pro edition.  This has happened to me before too and in fact, it happened again this morning.  I just X out of all applications and lock my computer at night and do not restart every night.  I did this last night and when i tried to open RC phone this morning it would not come up.  It showed in my task bar but didn't come up.  I had to restart my computer.  It works after a restart, but shouldn't be an issue in my opinion whether i restart or not.  i have also been getting crashes when trying to answer the phone periodically as well.  i keep trying to do the 10.3.5.34099 update (thinking that will resolve my issues) but keep getting an error message.  I am having my IT people look into getting the latest version downloaded.

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I also have this problem. RC ok after reboot but if minimised it is not possible to bring it back on the desktop.
I also run RCMeeting but that works fine

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