With Zoiper for iOS you can change the ringtone for every account.
The ringtone can be change by going to Zoiper -> Config -> Accounts -> tap on the account for which you want to change the ringtone -> scroll down until you find "Ringtone" .
Note that currently you can use only the built-in ringtones in iOS, if you have added any custom ringtones manually, they will not be visible or selectable. This is due to iOS limitations.