After 5 prototypes built and field-tested, here are the conclusions.

One thing wrong with our hardware setup was battery: a half-empty 1200 mAh battery doesn't give enough juice to power-up SIM 808. So we upgraded to 4400 mAh, and our device became fully self-sufficient.

A few bugs in firmware fixed, wake-up timer set to 4-hour period.

There's a couple of bad things specific to GSM networks and modules though.
While we cannot do anything to fix them, we can do a couple of things to live with them.

First of all, SIM cards have PINs. Makes sense for human-operated devices, but for stand-alone self-sufficient things, not so much. Bottom line, PIN has to be hardcoded in the firmware. Easiest way to get rid of PIN is to insert SIM into a smart phone and disable the PIN, exact steps are specific to device model.

Then, we used prepaid SIM cards of various mobile operators, and they all wasted all available credits on very first day.

You wouldn't expect sending a few kB of HTTP requests cost a few EUR eh?

Well, that's how bastards steal from their customers: you have to register SIM for data traffic, or else!
Registration itself is sending a specific SMS from device containing SIM card to some proprietary number.

Again, do that while SIM is in your phone.

But of course, that kind of issues is why we have LoRa and Sigfox and stuff.

In our case, GPS is must have, and HTTP is very nice to have, and SIM 808 has them both.

That would be all for the time being. You may still hear from us improving software eventually.