Customer relationships vary enormously from business to business. Some customers might see you as an extension of their business; others will treat your service as a transactional affair and barely remember you the day after they worked with you.
If you have over 500 customers/past customers but you don’t have an extensive account management or outbound customer service team, you might be struggle to find the time to maintain regular contact, to learn about their ongoing needs, get feedback about your service, and to find out how you can work with each other again.
Emails and newsletters do help, but they don’t give the immediate two way feedback that a phone call does on contacts’ current requirements and how they view your company. A personal phone call also shows that you care and are keen to continue working with them.