'3' but where in the world ('3', Hutchison 3G, has/operates networks in many countries, and the settings would be different for each country).
In any case, you can probably have the appropriate MMS settings sent to you as an SMS from '3' if you call their customer service. You may be able to order the settings from their web site, too.
Or you can go through
http://www.nokia.com/support as Nokia has at least in the past had a web page where you can order the settings to be sent for several phone models/operators.
And if you have a phone with the "Settings Wizard" application on the phone, you can try running it to get the settings that way, too.