Hi, When users log in to ECP they are able to change their address and phone number. How can I add more fields to this form? For example I would like users to be able to change the attributte "ipPhone", how can I make this appear in the account information form in ECP?

There is some related discussion here: http://social.technet.microsoft.com/Forums/en-US/exchange2010/thread/b491efb9-093d-4236-89a0-307041284765 But as far as I know, there is currently no supported way to do this.

Thank you for your answer, it appears to not be possible. If I can not use ECP, is there anything else I could use for this purpose? Right now it seems that my only option is to create som sort of simple self-service website to allow users to do this.

Yep. I think the self-service website is the best option at this point.