The other two OSs are Linux and Symbian-Series 60 which is owned by Nokia.
Although Sony handsets also run Symbian, they use the UIQ interface which will not be supported by Vodafone. Unfortunate then that Sony have just shown committment to UIQ by buying it lock-stock from Symbian.
UIQ is one of the User Interfaces from which handset manufacturers can choose when building phones on the Symbian OS. The others, Series 60, 80 and 90, are owned by Nokia. As well as supplying a third of the world’s mobile phones itself, Nokia also licenses its UIs to companies such as Samsung and LG.
Due to the large number of Nokias on the market, plus the extra S60 licensees, UIQ is a very distant second. Although UIQ has licensees itself, they are few and far between and include Motorola smart phones, and Ben-Q (oh dear). With Sony Ericsson only being fourth largest in the manufacturer stakes they have a lot of work to do to catch up.
Vodafone aren’t looking to budge. A spokesman said "before announcing this we did full evaluation of the industry" and went on "we remain committed to our three core platforms".
UIQ will remain as a standalone subsidiary of Sony Ericsson rather than a research department, and will continue to be licensed out to third partys on fair and equal terms. This makes sense as Sony will need to get as many UIQ handsets on the market as possible to get back on Voda’s radar.
The alternative is to launch a phone with S60 to stay with Voda, but that would mean licensing S60 from Nokia, it’s OS rival. Not the preferred option.