[throwaway283719]

Which other asset classes are you thinking of? I think that most transactions in cash equities, spot fx and futures are now done electronically.

I'm sure that a large proportion of derivative trades are still done over the phone, but that's because they tend to be (a) quite customizable and (b) quite hard to price.

It is incredible that treasuries are still traded over the phone, however, given how (relatively) easy they are to price. Even treasury futures, which are harder to price due to the basis and optionality, are mostly traded electronically, which tells me that there must be some parties with a pretty strong interest in keeping bond trading a "closed shop".