We are using the softphone that is installed on the computer. When we got the "not responding" issue we solved by following the steps I have written in the comment field.
https://community.mypurecloud.com/s/question/0D54400004u0aeA/agents-getting-forced-to-not-responding?s1oid=00DE0000000IvgO&OpenCommentForEdit=1&s1nid=0DBE0000000bloC&emkind=chatterLikeNotification&s1uid=00544000008d2C5&emtm=1498602918228&fromEmail=1&s1ext=0