Windows 10 Technical Preview for phones is now available to some Windows Phone users23
Since the apps that are available now support Windows Phone 8.1, if you do decide to install the Technical Preview there is a good chance that your apps will not run as smoothly as you are accustomed to seeing. If you have just one Windows Phone device, think carefully before installing the Technical Preview. We don't know how difficult it will be to rollback your update to Windows Phone 8.1 if necessary. You could end up losing the most basic capabilities of your phone, and not be able to make or take calls on it. Do you really want to take that risk?
The Windows 10 Preview is available for the Nokia Lumia 630, Lumia 635, Lumia 636, Lumia 638, Lumia 730 and Lumia 830. New features include expanded Action Center, Touch versions of the Office suite, enhanced speech to text capabilities and more.
If you do decide to install the Technical Preview of Windows 10 for phones, remember that you are being asked to provide feedback to Microsoft. That is technically the reason why Microsoft allows you to get the update in advance. No one is holding a gun to your head, but if you do follow-up with Microsoft, you are helping the company help you.
source: Windows, WindowsInsider via WindowsCentral