The final step in the signup process is the "IRL verification" where we try to make a connection to something less virtual than an email address. The easiest way to pass this step is to receive an SMS with a verification code.
It's important that you get the format of your phone number correct to receive the SMS. Include the country code and area code (cellphones usually have their own "area" code always included in the number). Don't enter any spaces or characters such as '+', only digits. Remove any leading zeroes in the country code and area code.
For example, let's say you live in Sweden (country code +46) and your cellphone number is 0708-123456 ("area" code is 0708 in this case) The number you should enter is: 46708123456
List of country calling codes at Wikipedia