I'd go with Ludo's suggestion. You could use the Custom Fields Power-Up to create a date field to store the last date you talked to a customer and a number field to count the amount of times you spoke to them.
Taking that a step further, you could use Butler to create a Card Button that updates the date of that field and increases the number by one every time you click it.
Here's our intro document for Butler along with the list of variables you can use:
If you had to thrive a new habit during a lockdown, what would it be? Trello
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