When I enter another (except my phone number) cell number, I receive a code via SMS or phone call, but when I enter my recent phone number, I face a message that indicates "ERROR".
Viber developers set up a page with this issue, more exactly with the issues you get when you want to register for an account. I recommend checking the page to see exactly what the correct procedure is to activate the account. As far as I know, you can activate an account only for one number. If you receive an error while activating another account, then the developers have to look into it.
Support page: https://support.viber.com/customer/portal/articles/1403939-solving-access-code-issues-#.VOb7rPmUe-M