Lava can be used to do functional tests on real hardware. It can be easily installed on PureOS, an FSDG compliant GNU/Linux distribution.
Labgrid has features similar to Lava but it is probably easier to learn because it's probably way easier to get started with it.
The OsmoGSMTester project is able to be interfaced with the Android RIL through ofono and can emulate a GSM network with the help of a compatible GSM base station or SDR.
This presentation from 2019 has many insights on what kind of issue we might expect in interfacing it with smartphones.
The Simtrace 2 project can be used, along with SIM card readers to programmatically feed a SIM card to a smartphone. This can be used to run test on real networks.
Serial port and USB:
Buttons:
Battery:
Antenna connector and GSM tower or SDR:See also the links inside the Serial_port article for more background on how to get serial port access on devices like the Galaxy SIII (i9300) or the Galaxy S2 (i9100).
See https://git.replicant.us/contrib/GNUtoo/test-infrastructure.git for the source of the file.
12:27 < juri_> I can confirm that the 4 pin battery connector available from amazon, marketed for the 'S4 value edition i9515' is capable of contacting the i9300's battery. I'll draw up a case for holding the battery and connector together next.