The phone number is to try and stop people signing up multiple times to get more free credits.
At least in the USA, getting hold of large numbers of phone numbers for free isn't easy.
At least in the USA, getting hold of large numbers of phone numbers for free isn't easy.
Even for a one-time-use for verification, last I looked for one-off number verifications, it was $1 per authentication; to be fair, didn’t search too hard.