I am currently trying to develop an application for Windows Phone 7.1 and above, and the main goal of the latter would be to edit Contact details, specifically the Phone Numbers. Is it even possible? I found out that it is impossible to edit or delete a contact from code, because these tasks are (by design) only available to the phone user. If this is true, is there an alternative way to do that? Will editing those contact details work if I develop it on PhoneGap?
It is not possible to edit or delete a contact from code. If it was possible it would be too easy to perform some malicious action and one of the guiding principles of the phone's design is that users data should be secure.
On WP7, it is possible to prompt the user to create a new contact (with details pre-populated).
On WP8 you can also work with a custom contact store which if fully controllable by your app but will not allow you to alter any details of the contacts that the user has added directly into their phone. Read more on MSDN.