There are a few potential issurei derabanon that may occur when using a smartphone on Shabbos. Such issurim may include:

Writing: Typing creates a few problems. The most serious being the melacha of kosaiv, as when one types it appears on the screen and this may be considered a form of kesiva. Additionally, typing on Shabbos the same way to type during the week is a problem of “uvdah d’chol”. Last, text that is typed remains as data on the phones hard drive and is theoretically permanently “written” on the hard drive as data (until it is removed) – this is a potential issur derabanon of writing (Kosaiv) according to some Poskim.

Sounds: Sounds are made when a user notifies or is notified of new messages, watches videos, listens to songs, etc. This is a potential issur derabanon of making noise (Hashma’as Kol) which is assur. Screen: Turning the screen on and off is may be similar to turning a non-incandescent lamp on and off – this can be an issur derabanon of making a fire Mavir or Molid and possibly Boneh, Soser and Makeh bepatish.

Misuse: On Shabbos, one may not engage in any business and weekday like activity such as business emails, stock quotes, shopping, etc