I am having the same issue as described in this post:
https://forums.verizon.com/t5/My-Verizon-Account/Collections-agency-amp-reported-and-added-as-deliquent-on-my/m-p/818467
I moved recently and had to cancel my service as fios was not offered in the new location. At that time I thought I was paid in full and the main landing page always said I was (showed a balance of $0.00) also I did not receive any bills during that time. Months later I checked my credit score and saw that it had a delinquency from Verizon. I went back to the page and it still said paid in full. After some digging through the payments section I found a small balance and paid it off. My credit score however is still much lower than before. Is there anything that can be done?