Hello.
What phone number format are you using? I can't exactly recall if the prompt shows just a blank text field, or you have a country code to pick first, but I found that to be the most common error.
Try using: +COUNTRY_CODE PHONE_NUMBER format, but without a zero for the operator code. For example: +381 65 1112222, where +381 is an area code for Serbia, (0)65 is an operator code and the rest is a phone number.
If you do have a dropdown to pick a country code, do 0651112222.
Also, try sending an SMS to yourself.