I just called the collection agency, and had a really pleasant conversation with the agent I got. She was friendly, clear-spoken, polite, and even funny. After I explained that both Verizon and I had seemed to agree I didn't owe them any more money, she said with a smile in her voice, "Now, I have to lay out all your options, and I'm going to guess you're not going to take this one, but the first choice is to make a payment right now for the amount listed."
She took down my info, and said since the debt was disputed, they'll send it back to Verizon, and I should get a letter in a few weeks. After I thanked her and hung up, I decided that wasn't enough, so I called back and asked for a supervisor, who I told exactly how helpful and pleasant Shirley had been. I can't imagine a collection agency gets a lot of compliments for their staff, and she impressed me enough to deserve one.