SMS Pager enhances your built-in SMS application by filtering incoming messages based on “key words” and/or by the telephone number the SMS originated from. Once past the filter the SMS Pager screen is displayed with 3 buttons, where a selection of responses can be pressed to acknowledge the received message and make a return call or SMS reply.
Huawei phone: you have to select SMS pager app as a protected application in order to run in background. Be sure app is not block by antivirus
Samsung S8 and similar: select Apps>Settings, Device maintenace, Battery, Unmonitored apps - add SMS Pager in unmonitored applications (https://www.g-rega-sp.si/SMSpozivnikSamsung.aspx)