It's a yes and no answer. Yes, in the technical sense that you were interrupted from your required rest. According to FAR 121.471 (e) you are not to be assigned any duty during any required rest period.
However, there is a loophole that most schedulers like to use. And the basic principle behind it is this....you answered the phone. You're not required to be in contact with the company during your rest period, however they just "happened" to get in touch with you.
In your situation, you had already been given a duty assignment for later in the day, so you're not really accepting a different duty assignment with the earlier flight. It's kind of a "touch and go" method of duty assignment, however it is legal. (forgive the metaphor) Next time just don't answer the phone until your required rest is up.