First the user must verify that the phone number that he / she has entered is correct. If the phone number is in itself wrong surely it will show the error message for SMS verification. The phone number contains two parts namely the country code and the phone number. The user must select the country code correctly in the box next to write his phone number of 10 digits without any prefix.
The second most important step is that the Wi-Fi, cellular data, 3G or EDGE must be enabled on the user's smartphone. If one of the services is not activated then it will fail to send the SMS verification.
The third step is to restart your phone. You should restore all the settings and make it ready for the verification of the number. This error may also occur due to an older version of the software installed in the phone. Also in this case it is recommended the update of WhatsApp Messenger.
Sometimes even when everything works well, they may work slowly. So when the time runs out, you must take a call. Immediately after a few seconds the phone ring, just answer this call and listen to the verification code.