I believe that you first need to add the (free) feature "HD Voice" to your account, either by calling Verizon or using the features page of My Verizon online. After restarting the phone, this will enable switching to Advanced Calling on the phone.
There is a FAQ here:
http://www.verizonwireless.com/support/advanced-calling-faqs/ and look at "How can I get Advanced Calling 1.0?".
Just note that there are some disadvantages:
- as usual, calls placed in a 3G area cannot get voice/data. They never could anyway on any but a couple of phones that hardly sold at all
- calls placed in an LTE area while you are driving will drop if you leave an LTE area rather than hand off to CDMA, as they do without Advanced Calling.
But, all this is in the FAQ.