Fix loss of voice after second call drops (on a second line) in case using multiple lines on unistim phones. There is regression was introduced in r391379.

Reported by: Rustam Khankishyiev
(closes issue ASTERISK-23846)
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Merged revisions 425667 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@425668 f38db490-d61c-443f-a65b-d21fe96a405b