i do not want my call que to go to voicemail even after 15 minutes

currently using call que groups and after 15 minutes of someone waiting on hold they are sent to voicemail.  it causes the customers frustration.  they do not want to wait on hiold for 15 minutes and then be sent to voicemail.  is there way to add more minutes or turn the option off so no one goes to voicemail during office hours