Navigate to the MCS Home page and log in, then click the cog in the top right hand corner of the web page to navigate to the configuration  section.

Go to Features/Phone manager/Client profiles

Navigate to the client profile assigned to the group of users you want to turn he feature on and off for. If you don't know then navigate to Users and Business Units and find out

You can also just turn on/off for all users, to do this go though the below on each of the client profiles

Click the green pencil on client profile you want  to modify, 

the below box will pop up and on the General tab you can tick/untick the enable chat feature

Click Save and you are done.