Calls - not possible to change the Status to Completed, Calls stay in open To Dos

Calls - not possible to change the Status to Completed, Calls stay in open To Dos

Hello,
I have noticed that it is not possible to change a status of a Call to Completed. Actually, it is possible to change it, but the changed status does not stay.
Scheduled and even made calls stay in open To Dos.
Can you please fix this?
Thank you 
Miroslava