noun
low-cost smartphone plan
Refers to an inexpensive mobile service plan, often from an MVNO, that provides basic smartphone connectivity at a reduced monthly rate.
格安スマホに乗り換えて、毎月の通信費を半分にした。
I switched to a low-cost smartphone plan and cut my monthly phone bill in half.
格安SIM specifically refers to the SIM card itself, while 格安スマホ often implies the entire service plan or the combination of a cheap smartphone and a low-cost plan.
Compound of 格安 (かくやす, 'bargain-priced') and スマホ, the common abbreviation of スマートフォン ('smartphone').