There's two possible reasons for this:
1. Make sure your number is in international format e.g. UK is 447712345678. Don't put "00" in front of the number.
2. We send our verification codes out with the from address as "TEXTBURST". Unfortunately some networks don't like words in the from field and so they don't deliver the text message.
Please email us at firstname.lastname@example.org if you're having trouble receiving your verification code.