If you’re using the circuit-switched mobile phone calls it’s unlikely you will be able to negotiate that; GSM compression is optimised for voice and not data.

If you have a phone or cellular dongle with a serial interface you might be able to initiate a “data” call which gives you a modem-like link at speeds much higher than running an actual modem on the audio path. Note that this kind of non-standard usage will stand out like a sore thumb so might not be advisable.

Well, up to 33.6k modem. 56k required cooperation between the telco and ISPs.