I don't have knowledge in electronics but starting to learn.
I did some projects with the RPI in the past and have some experience in coding.

In the past i purchased a device that do exactly as the title says.
Like this product on ebay:
Wireless Audio Voice GSM SIM Plug Socket

But it's not working.
It did worked in the past when tested.
Maybe the lack of support for GSM(3g), although it still should be supported in my country but maybe not in all areas.

So i thought of doing the same thing with RPI Pico or ESP32.
But the projects that i saw people make, don't involve making a call to another phone.
And i asked someone who said that its not as simple.

The device is really small to my surprise.
The LTE Module are quite big(compared to the device i bought) and require antennas.
I know because it's LTE and not GSM. But the board is also quite small(compared to RPI Pico or ESP32).
When i removed the device to take a look. I saw that it had a USB A port and the GSM module have mini USB.
And it seems strange that device that cheap that i bought like 5 or 6 years ago is able to achieve something that seems not doable with today RPI or ESP32.
What do you think those usb ports are being used for ?

Do you have any suggestion of how to maybe go about doing a project like this ?
I do have RPI (zero, 3, 4) and LTE Module. But i'm hoping to make something that is more small and discrete.

 

Welcome to AAC.

You are making a mistake in your analysis because of to things:

1. You are assuming that because the RPi and ESP32 boards have “powerful” MCU on them, that makes them suitable.​

2. You are neglecting to consider that on the one hand some engineers were tasked with making a cheap, covert device and on the other engineers were tasked with making a high performance device without those constraints.​

​

It shouldn’t be surprising the cheap, GSM bug does its job as a cheap bug better than a general purpose MCU development board and an LTE communications module.