I have the same problem on my iPhone 6. The 'My Verizon" app is the cause of the problem of my company's 8x8 VOIP app not working on my phone when someone calls me. By the time I get the first ring it already rang 5 rings at the calling end and flips to voice mail preventing me from answering it. I removed the My Verizon app and now it rings immediately like it is supposed to. That app is not going back on my phone. It's junk-ware.
We are sorry to hear about the issue that you were experiencing with your calls and My Verizon app. It's our goal to make sure you can use it anytime without any issues. A private message has been sent to assist you further.-Sean