This feature is located in the "Advanced Options" section. Please refer to the section "Creating/Editing a Click to Call button".
Scroll down to locate the following option :
"Message played when the user answers" : what the client will hear when waiting for the business to answer.
"Message played when the business answers" : what the business will hear when waiting for the client to answer the phone.
"Message played when the call duration limit is reached" : what both parties will hear when the call duration is reached and the call is ended.
Please note that if you have selected the call recording option (see the section "Call Recording"), you will not have the possibility to personalize the message played when the user/business answers as it is mandatory to inform both parties that the call is being recorded.