Shoyfers (shoyferim?) are referred to in the book as one of Sitka's most profitable exports (along with the three-wheeled truck thing that Meyer Landsman uses to escape the Verbover's compound before being rescued by Willie Dick. This suggests that the word was originally a name for a specific brand of cellphone that, in time, became a generic name for all cellphones, rather like the word "Hoover" which is now used for vacuum cleaners made by other manufacturers. It could, alternatively, have originally referred to those phones modified for use by the very observant, which do not operate (except for calls to emergency services) on Shabbat and, in the case of smartphones, prevent access to certain websites and services; later becoming a widely-used term for all cellphones (Landsman certainly doesn't seem the type to observe the prohibition of melakhot, after all).
The Yiddish Policeman's Union has been an enormously successful book which is still selling well, so with Yiddish being a highly adaptable language which is increasing in popularity among young Jewish people it will be interesting to see if shoyfer is adopted and used more widely.