“gezeira d’rabbonon”
There is no gezeira against smartphones. They didn’t exist in the time of Ravina and Rav Ashi.
MO and DL rabbis use modern technology to spread Torah. There is even one promient DL posek, Rav Aviner, who has published volumes of his text message responsa. I have personally had shilahs answered via text message. And the number of MO Torah shiurim on the internet is practically uncountable — YU alone has more than a hundred thousand shiurim on its web site. Think about that. Even were you to devote eight hours a day to nonstop learning just the YU shiurim it would take decades to learn them all, and you wouldn’t ever get through them because they typically add several new ones every day as practically every shiur there is recorded.
This represents a revolution in the degree to which Torah can be spread that rivals the introduction of the printing press! And just like printed books, there is stuff we shouldn’t see. But would anyone seriously argue that we should burn all our printed books and go back to handwritten manuscripts?
If you can’t be trusted to avoid the porn and gambling sites, by all means donate your smartphone to a shelter for victims of abuse and stay off the internet. But I would suspect that for most of us, a shiur with audio and text on my smartphone is a much better use of time on the subway than observing all the immodest advertising (and there is currently one series of ads on the outside of subway trains that is particularly bad). When I saw that on a train I just turned away and looked back at the Hebrew text on my phone and started listened to yet another shiur.