There is a famous dispute amongst the major previous generation poskim with regards to Electricity use on Shabbos. All the Orthodox Poskim agree that there is a violation taking place, but the question, as you ask it, is whether it's a Melacha DiOraysa or DiRabanan. Rav Moshe Feinstein, the major Ashkenazi Poseik in America of the previous generation, held that the Isur was an Isur DiRabanan. In contrast, the Chazon Ish, HaRav Avraham Yeshaya Karelitz, of Bnai Brak, held that it was a Milacha DiOraysa to use electricity on Shabbos.
However, since you were using a buzzing and beeping smartphone, the case may have been different. There is a Kula with regards to Milacha on Shabbos called "Grama", causation. This means that you didn't do something, but rather caused a pre-existing Muktza thing to be increased. However, this Kula can only be used in a true Shaas HaDchak. For example, at the Kosel HaMaaravi, there is a constant danger of bombs or gun attacks Rachmana Litzlan. Normal metal detectors buzz when a sufficient amount of metal passes through. This is completing a circuit, which is an issue that may fall into Boneh, building, or Make BiPatish, completing a job. The way around this is metal detectors that have a permanent electrical current running through them, which is displayed by a dial. When someone tries to walk through the detector with a sufficient amount of metal, the dial turns, and security can successfully apprehend the attacker (see the website of the developer, the Tzomet Institute, for more information {http://www.zomet.org.il/eng/?CategoryID=249&ArticleID=100}).
I would argue that the same is true with a smartphone that is already active. As long as you don't push any buttons, it would only be an issue of Grama, as far as I know. However, I recommend asking a Poseik or a Rabbi for a true Psak Halacha to this question. To end off, I would like to share a vignette my 9th grade science teacher, who also happened to be a member of Hatzala, shared with me: as part of his training, he had to take a course in the Halachos of Pikuach Nefesh Doche Shabos, the concept of life-saving overriding the prohibitions of Shabos. On the first day of the course, the instructor asked my teacher: if you were called into a situation in which you had to make an incision into a body, what would you do? My teacher answered by starting to give a whole Drasha as to what he would do: he would do it with his less dominant hand (LiAchar Yad), etc. When he finished, the instructor said: No, you idiot, Pikuach Nefesh! Do whatever you need to! I venture to say that the same concept applies here: if you are truly in doubt about saving a life or getting help in doing so, do what you need to do! Ask the question later, because more often than not, in these types of situations, there isn't enough time to ask the question.