Multiple reminders to the same person isn't much use if he's in hospital, and you're ignoring the out-of-office replies.
The time this happened to us, I was pleased the company looked at our website and phoned the receptionist. We paid the bill within an hour, and moved the responsibility to the finance and IT teams.
Maybe there are too many customers who just ignore bills for services they no longer want to use, I don't know, but given these companies usually have a sales team it would seem worth the time to make a call or write a personal email to an existing customer to maintain their custom.