ASCII is just a standardized table for which binary value that should represet 'A', 'B', ...

It has nothing with the electrical interface between a microprocessor and a GSM module to do.

The microprocessor has a supply voltage. It may be 3.3V or 5V or possibly something else. The processor "normally" emits data with the same voltage as it is supplied with, but a number of more advanced processors is using multiple supply voltages - for example a lower voltage for the kernel (to get a high speed with limited power loss) and a higher voltage for the I/O pins, to be electrically compatible with older-generation peripherials.

Besides the output voltages on the GPIO pins, the processor also have a voltage range that it accepts when receiving data. You may have a 3.3V I/O pin that is 5V-compatible, i.e you may be able to directly connect a output pin from a 5V chip to a processor driven by 3.3V. But since the processor emits 3.3V in the other direction, the processor may send out too low voltages to give acceptable noise immunity and detect levels for the input of an external 5V-driven chip.

Very few GSM modules has RS232 ports. They normally have I/O pins operating at the same voltage as the GSM supply voltage. If this is 1.8V or 2.8V is up to you to figure out from the datasheet. But you must verify all voltages in both directions. What voltages does the processor give out as high and as low. What are the max allowed voltages in on the GSM module input pins. What are the detect leves for a high and a low. What are the voltages the GSM module gives out on output pins for a high and a low. What are the max allowed voltage the processor pins accepts, and what are the acceptance levels for a high and a low.

This will tell you if you may connect the GSM module directly to the processor, or if you need any level converter chip, or if you can possibly just use a series resistor on the signals. Note that if you do have a GSM module with a RS232 port, then the signalling will not swing between 0V and VCC, but will swing between a positive and a negative voltage. This can not be fed into the processor, so in such a case you _must_ use an RS232 transceiver chip. This chip must talk RS232 levels with the GSM module, but must talk the correct digital levels (3.3V, 5V or whatever your processor uses) on the microprocessor side. Note for example the difference between a MAX232 and a MAX3232 - they are intended for different supply voltages.

But everything we are talking about here is completely outside the "microcontroller business". The rules would be the same if you interface two digital logic chips of different technology or of different supply voltages together - whenever you interface two "different" things, then you must figure out in what way they are different, and if they are similar enough to work (or even survive) direct interfacing, or if any glue logic is needed in-between.

So you'll have to set up a matrix of all combinations of min and max levels for low and high signals, and check if the combinations are acceptable, reliable and survivable.