There is also https://dash.by which works with many cheap OBDII devices, including Bluetooth ones. I was actually about to get started with them, but your setup is actually a cool idea. That is to say, you are combining an OBDII dongle and a cheap phone into what is essentially a single "device" which is actually internet connected.
My suggestion would be to research Automatic and Dash and see if you can improve on their setup by bundling all three components into one: try to actually create a device that combines and OBDII dongle, a GPRS modem, a GPS chip, etc. Also see if you can find and white label a cellular provider similar to RingPlus do you can do the whole thing as a service: $5/month plus a $100 device is not a bad deal.
I have one from 2008, which I don't use anymore. It speaks NMEA and it sends the data over GPRS. I guess there are 3G devices now. An advantage of GPRS is that it's everywhere (at least in Europe) because basically it's data over GSM. It's cheap. A data only SIM with 2 MB per month per year (yes MB) was 12 Euro per year. But you're not sending many positions and it's few bytes per position. I made a site like gping.io back then. Not enough customers but it could be different now.
A word of caution for any kind of GPS device, phone included. The GPS antenna needs to see the sky, best if a lot of sky. The roof of the car is enough to severely shield the GPS signal. You need assisted GPS then.
But you can hide your phone somewhere under the carpet or something and use an portable battery to have juice for weeks, this can work as a nice anti thief device. Automatic s' dongle can be easily located by a thief.
