Perhaps your mom thinks that once you are "independent" you may make some bad judgement calls? For example my neighbor added her 2 granddaughters to her cell account. She gave them specific instructions on how much they were allowed to use the phone (minutes, special ringtone charges, etc). If they went over budget she would turn the phone off. When they turned 18 they wanted their own phone so they could use the phone whenever. She didn't want them to get their own account because she knew if they didn't have someone to control their limits, they would run the bill up and not be able to pay. They would then ask her for money and she'd be out even more money than what she was paying if they were still on her account. Unfortunately that is what happened. When her granddaughters were on the account she paid a total lof $189 for all 3 phones. Her bill for her single line went down to $75, but the next month her granddaughters presented her a bill they couldn't pay for their phones. One bill was $360 and the other was $410. She's retired on social security and didn't have the money to give them. They ended up with a bad credit account at very young ages and it's hard to recover from that. She knew what would happen but they were to stubborn and immature to listen.

Since you are legally an adult, just sign your own contract with a cell-phone service provider. You will have the hassle of telling everyone you know and do business with you new number. It'll be 100% yours and there isn't anything you can do about it.