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Signal and Threema want nothing to do with WhatsApp
(www.androidpolice.com)
This is a most excellent place for technology news and articles.
You can buy a license directly from Threema via credit card, btc or cash in the mail.
I know this. That's why I wrote that it's too difficult to deal with cryptocurrency payments on their website. Most people won't do it. Other payment methods are not available from my country.