How did you book the ticket originally? Online via the Level site? The USA number for Level is (1) 646 585 9797. Note they only open 09:00 to 22:00 hrs CET. It may be that at peak times calls are falling through to Iberia's system, but I would keep trying.
Vueling do handle Level's webforms but they are actually quite responsive, so while I would not leave it with just a webform, I would do that as well. Their Twitter team often reassures people that this page is up to date:
https://www.flylevel.com/en/contact?sf104795513=1