This is almost impossible and similar difficult to answer as the question about ecological footprints and the question about impact of travelling.
Before we find out how much virtual water comes along with a cellphone we need to find out how to calculate the virtual water footprint.
You already mentioned the Water Footprint Network. Scientific head behind this project seems to be Prof.dr.ir. Arjen Y. Hoekstra at the University of Twente. In his book The water footprint assessment manual: Setting the global standard (PDF, pages 48-51), he writes:
In order to calculate the water footprint of the final product in a production
system, one can best start calculating the water footprints of the most original
resources (where the supply chain starts) and then calculate, step-by-step,
the water footprints of the intermediate products, until one can calculate the
water footprint of the final product.
It's four pages worth a read if you want to find out more about water footprints of consumer products. Hoekstra explains two different approaches of how to calculate the footprints.
But the essence is: You need data. For you cellphone you need to find out:
- all details on processes of the production
- all data of contained parts of the cellphone
- all energy and ressources used
... and maybe much more. To be honest, I doubt you will be able to research this on your own and cellphone producers wont give out such data to consumers unless they really have some kind of transparency drive.
To find out the exact amount of virtual water used in your cellphone you will have to start your own dedicated research project which will only work in coorperation with the manufacturer.
I hope this helps.