Better to call or leave a letter for tenants?

[Lue Brooks]

Hi everyone. I live in a duplex and rent the other house to tenants. I recently have let them adopt a dog who they keep outside constantly which is against the lease. Normally I’d be fine with that but he’s barking nonstop which is also against the lease. Tomorrow I’m going to be doing something about it. I just wanted to know is it better to call them and let them know and give them a warning or should I leave a letter on their door giving them their warning?

To whoever replies thank you very much in advance!