Back in the day dial-up over GSM was supported via Circuit Switched Data (CSD). However, many carriers discontinued support for CSD when it was superseded by GPRS and EDGE. Furthermore, even if this was supported, it would most likely be turned off by the government when they disable Internet access.

This leaves you with the GSM voice channel which is only 9.6 kbps. The voice channel is further impaired by the audio compression which makes it rather unsuitable for data transmission. You should probably not expect data rates of more than 2.4 kbps.

However, a voice channel is a voice channel. If nothing else you should be able to use a modem via an old school audio coupler to your head phone jack using a very low bitrate modulation. Theoretically you could do the same with an app if you can find or make a softmodem app.

Failing anything else, there are a number of TCP over sound libraries that you could hack to work over a telephone call.