When taking af Phone in Lync client, some time phone GUI not showing - so I cannot tranfere og close call 

Perhaps a USB issue?  I'd suggest replacing the cable first, trying a different USB port, seeing if another phone works, seeing if it's better from another machine, and checking to see if there are any antivirus or other applications blocking USB access.

It is Lync 2013 client on WIN machine not a IP devices.

Is it the whole Lync client or just the window or just the phone icon within the window? Is it just when you're on a call or are there other times as well?  How many users is it affecting?  Can you provide more detail?  If it's the entire Lync client, I'd suggest a fresh install and maybe checking it on another machine. 

User take incomming response grp. call and then got the call, but no Windows shows up. I cannot get customer to give the correct information about how many and how offen. He has got the strange no window one time in 4 month, but he sais that other has got same strange no window.

Can you get them to reproduce it in front of you?  I'm wondering if it's just hiding somewhere, just off the screen or something.  Can you get them try to tab to it or bring it up from the task bar and hit Windows-Left, Windows-Up, or Windows-Right to get it back onto the desktop? 

I cannot reproduce it, its comes and goes. I have a lync trace file from the desktop. I can see some errors in trace file but cannot see if this has some to do with this error.

I cannot get the trace file now - its on other machine. I get back with trace file.

Please keep an eye on it, have them notify you immediately when it happens and see if it's just a user issue.  Try out those Windows hotkeys in advance and see if you can find a resolution.

Try with the problem Lync account on another computer to check if this problem persists.

Test with other Lync account on the problem computer to check if this problem persists.

If the problem related with Lync client, try to uninstall and reinstall Lync client.

You might want to check the DPI settings on your PC. See the below post for reference as it worked for us.

http://social.technet.microsoft.com/Forums/lync/en-US/3d13cba9-26d7-46d7-822f-93d555682eda/incoming-toast-notification-not-displayed-at-150-screen-resolution?forum=ocsvoice

You might want to check the DPI settings on your PC. See the below post for reference as it worked for us.

http://social.technet.microsoft.com/Forums/lync/en-US/3d13cba9-26d7-46d7-822f-93d555682eda/incoming-toast-notification-not-displayed-at-150-screen-resolution?forum=ocsvoice

• Marked as answer by ERAVN 19 hours 12 minutes ago

You might want to check the DPI settings on your PC. See the below post for reference as it worked for us.

http://social.technet.microsoft.com/Forums/lync/en-US/3d13cba9-26d7-46d7-822f-93d555682eda/incoming-toast-notification-not-displayed-at-150-screen-resolution?forum=ocsvoice

• Marked as answer by ERAVN Tuesday, January 28, 2014 4:38 PM

You might want to check the DPI settings on your PC. See the below post for reference as it worked for us.

http://social.technet.microsoft.com/Forums/lync/en-US/3d13cba9-26d7-46d7-822f-93d555682eda/incoming-toast-notification-not-displayed-at-150-screen-resolution?forum=ocsvoice

• Marked as answer by ERAVN Tuesday, January 28, 2014 4:38 PM

You might want to check the DPI settings on your PC. See the below post for reference as it worked for us.

http://social.technet.microsoft.com/Forums/lync/en-US/3d13cba9-26d7-46d7-822f-93d555682eda/incoming-toast-notification-not-displayed-at-150-screen-resolution?forum=ocsvoice

• Marked as answer by ERAVN Tuesday, January 28, 2014 4:38 PM

I try to see NeXT time DPI