Is there an app that connects to your phone via wifi and let's you write sms's on computer with Windows 8.1 and then send sms via carrier network?
Something like Mightytext from android
There is an app called TextNow which has a Windows Phone and Windows PC modern app which does function similarly but it uses its own number to text via data, it's not exactly what the OP asked for though. And there are other similar apps too. And the service is US only I believe.