I have never gotten results by attempting to use a company's regular support channels. Best bet is to research who works there in a capacity that would actually be concerned about security issues.
On Nexmo's leadership page[0] I found their CTO, Eric Nadalin. A little LinkedIn search got me his profile[1]. Searching his name shows a lot of sites that would allow you to reach out to him (e.g. AngelList, Facebook, Twitter, etc.)
If that does not work, try reaching out to some of the companies that are Nexmo's clients. Even if Nexmo does not care, you can be sure that most of their clients will care, and they will definitely have the attention of Nexmo.