No analogue modems - which use special non-voice high frequency tones to communicate with one another - will work with any GSM network. Quite simply the reason is that the networks, being digital, have been programmed to only pick up or "sample" voice tone frequencies and discard non-voice tones. This is because the networks use a digital technique called Time Division Multiple Access (TDMA) to squeeze more calls onto one channel by essentially dividing the calling channel into a few discrete or "discontinuous" pieces. It achieves this magic by using a COmpressor or DECompresor (CODEC) to strip as much redundant information as possible before transmission, allowing up to 7 users to share one GSM channel simultaneously. Because it's been optimised only for speech, the CODEC that the GSM networks use assumes that only speech is being transmitted and is therefore unable to process anything else - like the frequencies or tones generated by analogue modems. Stripping bits of data from a modem transmission is of course suicidal for data comms since they require a continuous link. Hence you'll get very little if any data transmitted along the digital GSM networks using analogue modems. But if you do by some miracle manage to do so,
you're only going to be able to transmit at 75 bps, or about 0.8% of the
data transmission capacity of the network and an analogue modem.
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