WHMCS and our plugin attempt to format your input in such a way that it can be used by our API and other external systems. Sometimes this conflicts with your usual way of working.
The most common cause of a phonenumber being judged as invalid is because users add a country code to the number. WHMCS does this as well (using the country of the contact to get the countrycode) and this results in +3131...
In most cases removing the country code (and leading zeroes) from the contact is sufficient to get the phonenumber through validations.
Sometimes the error is simply due to a leading zero too many or a single number missing at the end. Also avoid using spaces in the number, this could cause issues.
If the cause is not clear, feel free to contact our support department. They can check the logs and see what is being sent to the API and possibly hint you in the right direction.