Preamble:
As an experiment I bought a relatively cheap GPS tracker that supported 3G (most of them at the time were 2G only).
After quick google search I found a suitable model GT06E from Concox.
The idea was I would implement my own server, as I do not trust 3rd party GPS tracking services (who would?), especially free ones ;).
I did not realise at the time what a mess the protocol is.
The “engineers” who wrote the spec for the protocol are crazy! They reinvented the wheel, which instead of tyre utilizes boots.