No, you have registered by signing up with Google and hence phone number is not mandatory for you. You don't have to share your mobile number to withdraw money.
Good to see that, but you need to do that (phone number), with the offer $50, that possible cheater could be created a thousand Gmail accounts (with .dot and + trick) to receive your offers, for example:
example@gmail.com
exampl.e@gmail.com
examp.le@gmail.com
examp.l.e@gmail.com
exam.ple@gmail.com
example+01@gmail.com
example+02@gmail.com
example+03@gmail.com
If you find it like that, you have to immediately ban that account asap.