Yes, I see they "answer" this exact questions hundreds of times a week over at the forum, but none of the answers are helpful.
The number I used does not meet any of the disqualifying criteria. AND, I don't even know that is actually the problem, because the mystery of the service is such that it refuses to actually let you know the error, if any. AFAIK the number is NOT a VOIP and it has not been used before.
More importantly, I don't have another number to use. How do they expect us to keep getting phone numbers to add to the system? Where am I supposed to get another phone number other than the one I have? Most of us, including me, don't have numbers from those services anymore.