I noticed that using document.location='tel:' + phonenumber; isn't working in iOS 5, this sucks because the prefered method is to launch that via a function tied to a button handler. If your using PhoneGap I have created a plugin to help manage this interaction natively.

Check it out here:
https://github.com/j-mcnally/PhoneGap-ios-PhoneDialer

hope that helps someone.

--Justin McNally