To configure an IAX account in Zoiper you would need to follow these instructions:
Zoiper 3 - Go to Settings -> Preferences -> Accounts -> Create account. Then choose IAX and fill the settings for Domain, Username and Password.
Zoiper for Android - Go to Configuration -> Accounts -> Add account -> IAX -> Select yes if you already have an account -> Manual configuration and fill the required settings for Host, Username and Password.
Zoiper for iOS - Open the Settings menu and go to Accounts, then choose "+" and select yes if you already have an account -> Manual configuration -> IAX and fill the information for Domain, Username and Password.
Zoiper for Windows Phone 8 - Run Zoiper and choose "Create" -> "I already have an account" -> "Manual" -> "IAX" and then fill the settings for the Account name, Domain, Username, Password, etc.