I wonder if telephone providers have some sort of recognition to detect people using (abusing?) a free call plan for transfering data. Certainly, if your "call" takes several days, that may violate some TOS.
(The use case I have in mind is to save money by using cheap prepaid SIM cards without an internet plan. It is purely for interest/proof of concept, not for scamming...)
Uhm, you are off on your numbers. GSM channel size is 200khz, but TDMA is used on top of that only leaving ~20khz for voice traffic (remember analog modems are “voice” traffic). So naively one might assume since 14,400 is under 20,000, it would be theoretically possible to achieve, but that’s not true in reality due to losses in the digital to analog to digital conversion due to noise. In the case of POTS, which has a max channel size of 64k, the theoretical maximum is roughly 38k (the reason 56k modems worked is due to the removal of the ADC conversion on the ISP side). Using a similar conversion for GSM, that gives you a theoretical maximum of 12k. If nothing else was a factor, then 9600 would be doable, but due to other issues with GSM, the real world experience usually was closer to 2400, sometimes getting 9.6k when everything was working great.
Source: I owned a dialup ISP in the 90s and routinely logged in from remote to manage user accounts from my 386 Thinkpad connected to a GSM phone. After selling the ISP, I worked for multiple years in the telco field, on the RBOC side (Southwestern Bell and later SBC), the cell carrier side (Voicestream and later T-mobile), and LDDS side (MCI and later MCI Worldcom).
Forgot to mention, GPRS uses the same time slots as voice calls do over GSM. Each time slot is still limited to about 9.6k useable data rate as with using a modem, but GPRS allows bonding to use more than one time slot. As such, using three time slots would get you 28.8k, using six gets you 57.6k, etc.
I suspect it won't be a very user-friendly experience, but as a POC? Sure!
1) per minute charge on the phone (typically 1-10p)
2) per monthly charge to phone company for line rental
3) per monthly charge to isp for access
4) per minute charge to isp while we were dialed in