Why Can't My Friends Join Steemit?

I have been inviting a few friends to join the steemit body. But that is proving to be a difficult task.

On the verification end , the email is verified but when they put in the telephone numbers, they are getting an "Invalid Number" message.

Therefore they cannot sign up...Do you have any suggestions as to why this might be happening.

When I joined Steemit a few years back, all that was needed was an email address.