G
Hi,
I like the way you think of solutions, but it is unfortunately not possible to call the device to send /set a voice message (in the category LTE, we only support NB-IoT or CAT-M1, only narrowband data services). Moreover, this is mainly used for uplink streams (ie, device to internet). For some carriers, it is possible to send a text message over the narrowband LTE, but I still would not trust the internal DAC to output proper audio/voice sound, meaning you would need an external Digital to Analog Converter to get the sound out.
In terms of alternatives, I cannot think of anything as practical as you mentioned with this hardware. You could make it output beeps through the PWM indicating information, and get the information on there through perhaps LoRa or LTE data..