I had this same frustrating problem. Customer service said I had to contact tech support and have my phone reconfigured from some outside source (garbage). I just simply shut down my phone for about 10 minutes and restarted it and my call wating is working now. I saw the suggestion on a previous blog and it did work for my phone. Hope it works for yours.
On line with Verizon, they are saying there is an update to be preformed. Waiting to see if it will work.... I'm confused, because the call waiting feature was there and worked before.
OK! It works, so just call 611 on your cell phone and they will walk you through it!!
I had to go to a Verizon store, it took about 20 minutes for them to figure out the problem, but they were able to fix it. I haven't had a problem since, and it's been about 4 months ago that the problem was taken care of.