What reminder apps are there - about each client?

Question

Android

The bottom line is that there is a customer base of 50-200 people. Each client needs to write in a personal (VKontakte) once every 2-3 days. Then get a response from him and write again in 2-3 days. And so on until the end of the paid period. Maybe someone knows what applications are for such purposes so as not to lose customers. And then the active ones, those who always write themselves and always answer in plain sight, and those who are passive or do not answer or promise to answer later, they are constantly lost and forgotten.

Answer 1

google calendar