Not sure why it's done but it seems that currently there is no way to register a phonenumber of the attendee.
I would say that it would be good to also add a way for adding phone-numbers. It's also not supported by default users if i'm right so that would be some added coding to be done.
When adding multiple-users at once it's a bit difficult to add but one addign users 1 by 1 then an option can be added what people can fill in. Thing is that for attending etc.. a phone number can be very usefull.