Step 1. Allow background activity
Open Settings → Apps → ProZvonki → Battery and choose Unrestricted. This lets the app receive calls and notifications while the screen is off.
One UI
On One UI Samsung moves rarely used apps into a "sleep" state and limits their background work — so call notifications disappear and reminders arrive late. Below is how to exclude ProZvonki from sleeping and, separately, how to set it as the default phone app (on some Galaxy models the "Phone" entry is hidden where you don't expect it).
Open Settings → Apps → ProZvonki → Battery and choose Unrestricted. This lets the app receive calls and notifications while the screen is off.
Open Settings → Battery (Device care) → Background usage limits → Never sleeping apps and add ProZvonki. Also make sure the app is not in the Sleeping or Deep sleeping lists.
In Battery settings (⋮ menu → Settings) turn off Auto-disable unused apps and Put unused apps to sleep. Otherwise, after a few days without opening it, the system will put the app to sleep again.
Open Settings → Apps → ProZvonki → Alarms & reminders and grant the permission (Android 12+). Make sure the app's notifications are enabled.
Open Settings → Apps → ProZvonki → Set as default → Phone app and pick ProZvonki. On some Galaxy models the "Phone" entry is missing from the general "Default apps" list — in that case set it here, from the app's own page.
About the default phone app: if the "Default apps" screen has no "Phone" entry, that's a One UI quirk, not an error. You can set the phone app from the app's own page (step 5).
FAQ
Add it to "Never sleeping apps" (step 2) and turn off "Auto-disable unused apps" (step 3). These two settings solve the sleeping problem on One UI.
That happens on some Galaxy models. Open the app page: Settings → Apps → ProZvonki → Set as default → Phone app — and pick ProZvonki there.
Check that the app is set to Unrestricted under Battery (step 1) and that exact alarms are allowed (step 4). Power saving mode also delays background tasks — keep it off if you can.