Can I have different default phone numbers for different actions?
I don't think you can set separate default numbers.  However what you could do is set a shortcut to message that contact and leave the default number empty so it can prompt you when you call.  To set a message shortcut: long press on the Home-screen, select "Shortcut", then select "Direct Message".  This will launch your default messaging app to send a message to that contact to whichever number you set when you create the shortcut.



Hope this helps