Short answer
Nexicall schedules call reminders as notifications on the phone itself. Android's battery optimisation — and on some brands a separate autostart permission — can delay or drop them. Excluding Nexicall from battery optimisation and allowing autostart resolves it in most cases. A reminder also needs a time on the follow-up, not only a date.
Android suspends apps it believes you are not using, and most manufacturers add a stricter layer of their own on top of Google's. That is genuinely good for battery life and genuinely bad for anything scheduled to happen later. A reminder is scheduled work, so it is exactly what these systems postpone — sometimes by minutes, sometimes until you next open the app. This affects every app that reminds you of something, which is why you may have seen the same advice for calendar and alarm apps. An app cannot opt out on your behalf; the permission has to come from you.
A follow-up needs a specific time. If you set only a date, no reminder is scheduled on the phone for it — those are covered by the daily summary instead. Only your nearest upcoming reminders sit on the phone at any one time, so a follow-up months out is not pending there yet. It becomes a scheduled reminder as it comes closer. Reminders are local to the phone. They stop if Nexicall is uninstalled and they do not move to a new device by themselves. The follow-ups are stored on the server, so they come straight back when you sign in again.
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