Phones get static and drop calls

We're using an AVAYA S8400 server going out to 3 lines:
2 PRIs for local and long distance calls
T1 for International calls

2 weeks ago users started to complain about static on the line on local and long distance calls. At first when it was one or two users I thought it might be the phone though you expect a failed hardware to fail at all times and the static was random.
Then more and more users complained and some even added a new complain about drop calls.

In order to isolate the problem I have routed 2 heavily used area codes to the T1. Since no one reported problems on International calls it was a great way to find if the problem is local, equipment or infrastructure on my end or is it PRI\Verizon related issue.

Verizon which run both PRIs tested them after hours and reported back that both lines are clear.
At the same time no one complained about static at the 2 isolated area codes.

As the weekend passed we came back Monday morning to a clean line. No static. No drop calls. I changed nothing and yet the problem seem to be resolved...

Before I close this case I had to switch the 2 area codes back to the PRIs and make sure they are still clear. 4 days later the lines are clean.
To be on the safe side I asked Verizon to test the local equipment (that would be the demarc in my server room). Last night when the technician finished testing he confirmed it is clear.

While no one can explain it there is one last possible option, something we'll never know but can always blame: The building lobby is under renovations.
We used to have our office downtown where the infrastructure is old and fail whenever it's raining. Whenever we saw a Verizon truck within few blocks we anticipated phone or Internet problems. Unfortunately we where right most of the time. Taking this experience under consideration it is more then reasonable to think this option is possible. But as I said, we will never have an real answer.