I have worked windows phone 7, but not 8 so I can't directly help. Maybe if you give us a bit more information we could figure it out together. What kind of binding are you using to connect to PI Web Services. What is the exception message your are seeing. I know that a lot has changed so I'm not surprised that you might be having issues. With WP7 you can only bind using basicHTTP, similar to silverlight. I think that is still the case with WP8 from what I just checked doing a google search. Anyway, posting more details will help, along with any source code you want to share. I can also try a test project to see if I get the same message.
Thanks a lot for your information. Now we've switched to use the sample web config named web_config_basic_no_security, and everything works fine.
We will try the other config files.