Every business already has a follow-up system. It usually lives in someone's head, or in a pile of starred emails nobody else can see. That pile is doing real work — it just is not written down anywhere.
Write down what the pile already does
Before adding software, describe the pile out loud: what goes in, what makes something come back out, who checks it and how often. Most of the time this takes less than five minutes and reveals the actual rule already in use.
Then decide if it needs a tool
A written rule that one person can follow does not need a CRM. A rule that only works because one person remembers it is a risk, whether or not it gets software — and that is a different problem to fix first.