Your mobile number is required to link the OK app to your mobile phone. This is also required so we can send you a text message with a verification code while installing the OK app. Phone numbers not verified within 5 minutes will be deleted. This means you have to try again.
You can change or fill in your mobile number in your settings. From the overview screen, click on the gear icon in the top right corner. Under the header 'Me' you will find, among others 'Mobile number'. Click to fill in or change your mobile number. Then click 'Save' in the upper right corner and accept, after receiving, the confirmation-text message. Mobile numbers that are not confirmed within 5 minutes will be deleted.
[More about what to do if you do not receive confirmation-text messages]