If you are looking for a secondary number, TextNow is one of your best options on Windows Phone. Before update 2.0, you could send and receive texts for free, and now you can do even more.
New in 2.0:
Make/Receive Phone Calls – Use your TextNow phone number to make and receive voice calls for free!
Voicemail – Give your callers the opportunity to leave you a voice message if you happen to miss a call!
Messaging Improvements – View your call log with a contact within a conversation window.
Various other bug fixes and UI improvements.
Calling using TextNow requires credits (1 credit per minute), which can be purchased or earned through redeemable offers. You will recieve 5 free credits upon downloading the app. All texts are free and not based on credits.
Disclaimer: Calls and texts can only be sent to Canada and the USA.