While referred to as such by Meagan Marie before, Sound Designer at Crystal Dynamics, Alex Wilmer, confirmed in writing today that the "Ringtone" available at TombRaider.com is the main theme for the upcoming reboot of the series:
"It is indeed the theme. It was very important for us to reveal it in this way." - Alex Wilmer
Additionally, Wilmer intimates his desire to share more about the music of Tomb Raider (2012) on the official Tomb Raider forums in future. The piece was performed by in-house concert violinist Rob Cheng, directed "emotionally" by the yet-to-be-announced composer.