Many users are reluctant to provide a phone number - for good reasons. Unless this is part of the registration process and already accepted by the users.
Indeed virtual/disposable phone numbers are readily available, even for free. Phone verification is not foolproof.
Have you tried available options like captcha ? Unless the tokens have redeemable value that is not negligible, you may be worrying too much.
What you should probably do is monitor the number of requests/day/IP address and there should be some form of rate-limiting. If many requests come from a single IP address this could be a sign of abuse.
You could also delay the granting of the tokens - subject to a manual overview. What you need is some kind of page/report you can run on a daily/hourly basis, that shows E-mail addresses, IP addresses etc, and highlight duplicates and suspicious signups. And then you release the credits in one click, except the suspicious entries you want to investigate further. For this, you use checkboxes so you can tick or untick specific users.
It is a slight inconvenience for users, but on the other hand this could act as a filter and help attract users who are motivated enough and willing to wait for a reward.
I think that one way or the other, you need to monitor the activity. At a minimum, you could get a notification by E-mail every time someone signs up to your site. If there is a sudden influx of registrations you will quickly notice.
If you remove the instant and automated allocation of tokens to users, there is less of an incentive for people to cheat. Also, they are alerted that registrations are being monitored and reviewed by a real human being. This is a put-off for profiteers.
That's the reality of business, not everything can be easily automated. The solution does not always have to be high-tech.
E-commerce ventures in the real world also have employees who spend time behind the scene vetting the new users, and also the not so new when for example certain orders are deemed to be out of usual patterns and trigger an algorithm. Sometimes they even pick up the phone and call customers.
It's all about risk management.