Hello,
Zoiper for iOS supports a Number rewriting feature that can rewrite the prefix for international numbers. You may configure it in a way where the numbers without international prefix could be rewritten with the proper format for your country and the ones with an international code to be rewritten with +, 00, or prefix by your choice.
To make the configuration you would need to open the Number rewriting section from the account configuration and under "Number rewriting" to select the proper country and the prefix you want to use for international numbers.
Please note also that for the moment number rewriting with multiple dialling rules is not supported in Zoiper, but it is planned for the future releases of Zoiper.