Just stumbled upon this thread and wanted to suggest you to try our (currently free) application Prefixer. You'll find more information about it here: Prefixer - 253ºBelow
Prefixer will automate the need of manually adding a prefix, suffix or making other changes to the numbers you dial. There's no need to edit your contact list either! You define your own calling rules and let Prefixer fix outgoing numbers according to your specifications. All this is done on-the-fly, in the same second the number is dialed!
Prefixer comes with a number of options already today, such as setting up rules that are valid only certain days of week, specific times etc.
For the next version (coming soon!) we are working on adding conditions like "number in contact list" and "contact in specific group"!