one signal:

After several years of rapid improvements, notifications are now at a point of maturity and broad availability to be the primary way for apps to deliver time-sensitive messages to their users. The advantages over SMS are severalfold – wider platform availability through both mobile apps and web browsers, richer content, better deliverability, greater interactivity, and more user control. It’s now only a matter of time before app-to-user SMS goes away entirely.
 Nevertheless, although notifications are more reliable than SMS, they’re not as reliable as they can be. The problem is due to device manufacturers not realizing just how much users rely on notifications throughout their daily lives.
 In a misguided attempt to increase battery life (or just the appearance of it in benchmarks), many device manufacturers have modified the Android operating system in ways that interfere with prompt notification delivery. Below is a list of affected manufacturers and devices we’ve seen here at OneSignal, along with the steps users must take to fix the problem: