On your iPhone click HERE to download Zoiper form the AppStore.
You will need to contact iTel Support for the following information:
Step 1: Open the app and press settings.
Step 2: Select Accounts.
Step 3: Tap the + sign to add a new account.
Step 4: Tap yes for having an account.
Step 5: Tap manual configuration.
Step 6: Tap SIP account.
Step 7: Fill in all the SIP Options. Account name can be anything you want. Caller ID is optional and can be your cell number or the number associated with your extension. Tap register when done.
Step 8: If everything was good and it connected to the server you will see Registration Status: OK. Tap Accounts to go back.
Step 9: Repeat Steps 3 through 9 for each account you want to add. If no more accounts to setup, tap Settings to go back.
Step 10: Tap Dialpad to go back to the main screen.
Step 11: At the top here you will see your account name and ready. This means everything is good and you can start using the app.