All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
said: “The pen is lifted from three: an insane person until he regains reason, a child until he reaches puberty, and a sleeping person until he wakes up." [Abu Daawood, At-Tirmithi, An-Nasaa’i and Ibn Maajah] Ibn Taymiyyah
said: “The scholars have unanimously agreed to its acceptance.”
Since your promise to your parents was made before you reached the age of puberty, then you are not sinful for not fulfilling the promise.
Even if your promise to your parents was made after you had reached puberty and they do not ask you now to give them the cell phone, then you are not sinful, Allaah willing, for not giving it to them; because if someone is promised something and is then content with the unfulfillment of the promise, then the one who made the promise is not sinful.
Allaah Knows best.