Found a number, for "Payward, Inc." aka Kraken: (+1) 650-434-2889 (https://www.bloomberg.com/research/stocks/private/snapshot.asp?privcapId=260965170) - it's a Google voice number with an anonymous outgoing voicemail message...

Here is one other number they apparently used in the past, another anonymous mobile number: (+1) 916-267-4413

How and why can they so successfully hide their phone number? 

It's easier to find a working number for some lawyers suing them, than it is to find a working phone number for them: http://www.almcms.com/contrib/content/uploads/sites/292/2017/07/1.-2017-7-3-CLASS-ACTION-COMPLAINT-Clark-et-al-v-Kraken.pdf

Also nice to know: https://digiconomist.net/fraud-risk-assessment-kraken