If you don't get your mother's permission or authorization, using her mobile phone card may lead to the following consequences:

Invasion of privacy: You may have access to your mother's private information, such as phone records, text messages, social media accounts, etc.

Expenses: If you use your mobile phone card to make phone calls, send text messages or spend data traffic, it may lead to additional expenses, which will be paid by your mother.

Security issues: If mobile phone cards and mobile phones are not protected, information may be leaked, stolen or hacked.

Therefore, I suggest you get your mother's consent and authorization before using the mobile phone card, or buy a mobile phone card yourself.