When we were little my mom made a deal with us.
"No matter what,happens if you tell me the truth I will not be mad. I may be hurt or upset, but I will not be mad at you."
To this day she has kept this promise.
Listen to what your daughter said. "I did not want to get in trouble"
Ok, so a week without the phone for talking on the phone late at night during the summer is appropriate. Fine. Since you have a rule about this, but begin to consider, she is young, it is summer, 15 year olds can stay up late and still function the next day.
Is your daughter overall a good kid? Does she make good grades, involved in outside activities and keeps up with her responsibilities? If so consider allowing her some privileges.
In our home when our daughter was in high school, we did not have a bedtime, we had an agreement. Stay up as late as you want, but you must be awake and functioning for the next day by.... 9:00am in the summer and in time to be ready for school and functioning in the mornings.
This meant she could talk on the phone, read, be on the computer.
During the school year, gasp, same rule as long as her grades stayed up and again she could function for the full day.
Your daughter is now old enough to figure out how much she can handle without sleep. When our daughter was in college, there were many, many nights of no sleep, not partying, but studying and writing papers. She knew what she could handle and was mature enough to make these decisions, partially because while in high school, she had learned her limits.
Time to take a step back and begin making the rules, with your daughter. Each can state their reasons, but if your daughter does not keep up with her promises, you start pulling back on some of these privileges.
My nephew will be a senior in August. He is a huge athlete,varsity quarterback, , belongs to clubs, volunteers and makes good grades. He does not have cell phone hours, but he has learned he has to be asleep by a certain time to be at his best to be able to keep up with his responsibilities. When he makes a poor choice, he takes responsibility, this is something he has learned by testing him self. No one to blame but himself.
Hang in there mom, she is growing up, needs guidance, but she also needs to win her own choices. Allow this now while it s safe and at home, not when she moves away and you are not there to make suggestions or observations.
I knw this is hard. But it is part of preparing her to be without you un a very few years.