2018 is when the PSTN will be closing according to AT&T. So start planning for it - whether that is (1) ride it out; (2) some exit strategy; or (3) Pivot.
Pivot means change your business model. For some, it is easier to start another business altogether than to pivot the existing one.
Either way a little planning can make all the difference in the next 5 years.
And that is just banking on it not being escalated by storms and other happenstance. Some say WebRTC may hasten this along since anyone with a Chrome browser can make audio and video calls now. Vonage utilizes WebRTC on its mobile app.