Hi Klaus
as far as i know any modem will need external MCU, for example to answer the call in the first place, then get the AUDIO data. unless we use programmable GSM module like A9 Ai Thinker ( i am planning on buying this type ) when you can control all the process inside. what i need is a bridge between MCU and GSM modem. i was saying i am going to need either I2S interface or using ADC with I2S to microcontroller.
if there is a cheap GSM module with I2S output, then i don't think i need external (decent) ADC/DAC to get audio data. but if i none of them were cheap, then perhaps i really should considering ADC/DAC with high speed interface like I2S cause i need to retrieve the audio in and feed the audio out.