As you've noticed, 2-year-olds are striving for independence. They tend to ignore their parents' no's because they're determined to assert their own will. Their motor and language skills are booming, too, which makes them perfectly capable of both running away from you and loudly proclaiming that they're unhappy with (or simply unwilling to abide by) your request. As frustrating as this may be, take heart in the fact that your child is practicing her new skills on the person she trusts most — namely you. What's more, she's still a long way from being able to control herself. So when you tell your child to stop playing with the telephone cord, she may be able to think of nothing but the cord and find it nearly impossible to put down.
When you find yourself in a moment like this — and you will — be patient. Distract your child from trouble whenever possible, and provide a stimulating, childproofed environment that gives you plenty of opportunities to say yes. Then choose your battles carefully and focus on the transgressions that are truly worthy of reprimand — such as running into the street or pinching her baby brother. When you do say no, make sure you mean it.