I also have it fixed, but the phone app goes into landscape mode when the keyboard is out (as it should) so maybe when you get a phone you slide in the keyboard?
I believe it does work when you lock it to landscape mode (at least for me it does). This also gets rid of the rotation when opening/closing the keyboard, so I like it better anyway.
I have this problem as well.. I generally end up hitting the text button instead of the answer button... grrrr. I have set mine to portrait only for the phone as well and restarted but when ringing it still rotates.... I would love to have the ringing screen locked to portrait only.
Thanks for the comments and good to see it happens to other people. Seems like an oversight in code not really a bug and there is a workaround which is not ideal which is to lock the phone dialler in Landscape or portrait. I hope this is something that's fixed in next release. (Rather like being able to recognise international number formats with brackets properly, ho hum, one day it will all be lovely)