Many smartphone owners these days claim the least used part of their smartphone is, actually, the phone. With data and the internet, it may be possible to ditch that cell phone in favor of something less expensive.
First you'll need a mobile hotspot to supply the data. Options include Virgin Mobile Hotspot, Verizon Wireless, an iPad with cellular data, or FreedomPop. You'll also need a device to use the data on, which could be an iPod Touch, tablet with Wi-Fi, or even an old smartphone.
You'll probably benefit from having a number that people can call you on. Google Voice is a good place to get a free number, with access to calls and voicemail on the web. Google Voice can also be used to send free SMS (text) messages! The downside to this is that Google Voice does not support MMS, the feature that allows texting photos or videos. It's also US-only.
Next you need apps to handle making and receiving calls. You can access your Google Voice account on a mobile device using the Official Google Voice appon iOS or Android. Unfortunately, it isn't possible to receive incoming calls with the official Google Voice app. For this, you'll want to have the Talkatone app, available on both iOS and Android.
For a more detailed explanation of how to get this to work, watch this episode of Know How.