Fix issue with no sound in both way in case of previous call to chan_unistim phone was canceled. 

(related to ASTERISK-20183)
........

Merged revisions 391379 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@391380 f38db490-d61c-443f-a65b-d21fe96a405b