If it was mandatory you wouldn't have gotten an email. The phone itself will have a pop-up telling you to update the phone and it will give you a brief details about the update.
Most likely spam. You should, however, keep your phone and apps up to date at all times. You can go to Settings on the phone and run the software updater contained within. For apps, update them through Play Store.
It's not for the phone, it's for the mobile app from Verizon, that's what I umderstand it to be. I get other updates when app need them, but this came from Verizon. mmobile.updatever2016.us/update/?id=0012401419....then my number
Scam site. Ignore it. All updates for the Verizon app will come through the Play Store.
If you're getting pop-ups that deliver these ads/scams, start looking into what app is causing them. Any application you have installed with the permissions of "Runs at Startup" "Draws over other apps" and happens to be a free app from the Play Store, should be seen as suspect. You can also run MalwareBytes (from the Play Store) to see if that catches anything malicious with any apps.