I have the same issue with my fop2 for a while now. But I have finaly found out how to reproduce it.
User has to fail to log into the fop2 one time and just after that type correct password and log in. After picking up any number from phonebook, user gets double call on his IP phone.
Any solutions for that besides "don't mistype your password" anymore?
I appeared on various versions of chrome. Also FOP2 versions were changed over the time.