Signal is a cross-platform centralized encrypted message service developed by Signal Technology Foundation and Signal Messenger LLC. Users can send one-to-one and group messages, whichcan include files, voice notes, images, and videos.
In a short words, Signal APP allow encrypted person to person message,similar like Whatsapp/Telegram, you can communicate with other people without worry about your information disclosed by the APP owner or your local government.
At this moment, this app is free to use. So sometimes the verification code is delayed across the world.As you can see from their twitter post as blow.
What you can do is you can leave message to them at their twitter or their official contact information at their website.