Facebook ask me for each confirmation click (for sending a message or otherwise) to do a safety test.
I am entitled to the eternal and annoying message:
Please check your account.
Your account must be verified before you can perform this action. Please check your account by adding your mobile phone or credit card.
I'm personally not at all motivated to send my phone number.
Apparently Facebook is asking your phone number, to send you a code via SMS.
Then you go to "Account" "Mobile" "Confirmation code" and you type your code. Normally it works ..