[123456]

Did you try this:

iPhone
Tap 'Settings'
Tap 'Phone'
Tap 'SIM Applications'
Tap 'Callback'
Tap 'Callback mode'
Tap 'Callback 4'
If you have followed the above instructions and cannot make outgoing calls please tap 'Callback 5' (instead of Callback 4).

or

If the call back does not occur, or if you hear a message saying the call cannot be completed, your handset may not be fully compatible. To resolve this problem, all you need to do is enter the number you wish to call prefixed with *126* followed by country code, area code (without the leading 0), phone number and #.

Example 1: if you wish to call 03 9010 0225 in Australia, you would dial *126*61390100225#
Example 2: if you wish to call 0207-943-2772 in the UK, you would dial *126*442079432772#
Example 3: if you wish to call 213-337-5555 in the US, you would dial *126*12133375555#

Source: http://www.ekit.com/ekit/MobileInfo/help_compatible