Carrying a Device on Shabbos
The Sheiltos in Parshas BeShalach (Sheilta 47) sources the commandment of mutkza from when Hashem instructed the Jewish people to prepare the proper amount of mann before Shabbos. Since only the items prepared for Shabbos are permitted, items not prepared cannot be moved on Shabbos. Additionally, the Gemara in Eiruvin (17b) derives the issur of the melacha of hotzah; carrying from public to private domain or vice versa is forbidden. A common question is whether healthcare personnel are permitted to have a phone or any communication device in case of emergencies.
Rav Moshe ruled (Igros Moshe O.C. 4:81) that radios were permitted amongst hatzalah personnel as it was considered a tachshit-an ornamental item. It is an item that indicates that one is an altruistic person who helps others in need. However, a smartphone does not fulfill this purpose, and it would seemingly be forbidden to be carried around.
However, the Nishmas Avraham quotes Rav Shlomo Zalman Aurbauch (301) that one is permitted to carry a device but must do it through a shinuy (an unconventional method of carrying), as well as stay away from an absolute reshus harabim (public domain). However, in a life-threatening case, one is permitted to carry it regularly, even in a public domain. This ruling was specifically in a scenario without an eiruv. If there is an eiruv, carrying a phone without a shinuy is permitted.
Adapted from medicalhalacha.org