How did gmail / google registration process tried to prevent the misuse / automation of the accounts …

orange and gray train

My three neice were trying to create gmail accounts for themselves.
When the first one was trying to create the account the phone number for the recovery of email or the phone number when creating the account was optional.
But when my second and third neice tried to create the gmail account the phone number became mandatory when trying to register.
I believe it was to prevent the misuse / not a bot that was trying to create accounts.

How did gmail / google registration process tried to prevent the misuse / automation of the accounts creation?

1- I cleared all the cache, broswing data, history , cookies.

2- Changed the mac address of the machine.

3- Tried to use different ISP service.

none of them worked(I expected the phone number to be optional).

Still the gmail was asking the phone number- ie mandatory.

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