Comment by goatsi

It's an anti abuse method. A valid phone number will always have a cost for spammers/multi accounters to obtain in mass, but will have no cost for the desired user base (the assumption is that every worthwhile user already has a phone).

Captchas are trivially broken and you can get access to millions of residential IP addresses, but phone numbers (especially if you filter out VOIP providers) still have a cost.