1-800-922-0204 and say agent to get a person. You might have to enter the phone number that was previously associated with your account.
It appears you can contact them by dialing 1-800-922-0204.
Press # when asked to input your old Verizon number to indicate you are not a Verizon customer.
Press menu option 4 to indicate collection notices or to pay a final bill.
You could also stop by your local Verizon Wireless store.
There is also Paying Your Bill Online