IIRC, the phone stack live completely outside of Symbian itself (I'm not sure how this looks in something like Android). So each vendor attempting to deploy a phone would have to provide their own telephony stack... I worked on at least two memorable projects where they just couldn't make that work.

Now Nokia has put out hundreds of models of S60 phones, each largely sharing components. I would imagine that their telephony layer was rock solid. I have no idea if the assertion in the article is true or not, but it wouldn't surprise me. Nokia has always "gotten" the phone side... it's the UX side of a smartphone that has always vexed them.