This typically means one or more of the following:
- Access the text messages on the device.
- Be notified when new messages arrive.
- Send messages without the user having to directly do something.
For very good reasons this isn't possible.
Those good reasons are:
- to provide a predictable experience to the user/owner of the phone (so they know what messages are sent and what data apps have access to)
- and to prevent malicious activity
But, you may argue, "I'd never do anything malicious".
Hopefully that's the case, but it's not possible to guarantee that everyone will be so good.
Just ask some android owners who installed applications only to find expensive SMS messages showing up on their bill that they had no record of:
While in this case the issue has been addressed and those affected will be getting their money refunded, stories like this hurts the platform. While Microsoft are busy trying to grow the platform and attract new users, stories about the security of the platform and how people with those phones can end up with unexpected charges on their bill if they install apps would not be helpful.
While you can't read or be notified of new SMS messages you can compose one for the user to send. See "How to: Use the SMS Compose Task for Windows Phone" for more details.