First and Foremost the "trick" you are using is an exploit of your ISP's poorly planned infrastructure. As such it is most likely just as illegal as joining your neighbors WiFi because they left the default password enabled.
Why it works:
Your ISP is only monitoring HTTP/HTTPS traffic either via a transparent proxy of there own, or deep packet inspection. For whatever reason, they are not monitoring (or more likely monitoring but not charging) for other forms of network traffic ie VoIP services. This means if you setup your device to tunnel its traffic through to an external proxy via sock5 or socks5 over ssh ... your packets will be logged as non-HTTP(s) traffic and thus you will not be charged (yet).
Why this is Dangerous:
Also, as I mentioned already ... use of this exploit is illegal. There is a very high likelihood that your ISP is tracking this bandwidth and while it is not currently charging you for it ... could retroactively charge you for it in a future bill.