What Constitutes Stalking/Harassment

Last week I got a call from Hammond Pole, telling me I owed Vodacom money, actually Vodacom owes me money as when cancelling a contract I paid 5 months penalty instead of 4 months.

However instead of sorting out the issue, I have been bombarded with emails, sms's and whatsapps.

How many emails, sms's and whatsapps do you have to get in a day to constitute harassment?

I am grateful for the mute button as I only have to delete all the messages once a day in one go.

Is this behaviour professional? Not in my book, its tedious and boring and as Trump would say "Sad"