This (should be) easy and fun project and i think the biggest challenge is going to be to make the thing look and feel like a brick.  I spent a couple of hours designing the following daughter board for the Maduino:

..... It's just a load of switches for punching in telephone numbers and some LEDs and a buzzer. There's also a nifty I2C connector in case some peripheral like a screen needs to be attached. The Maduino has an onboard MCU which can be programmed with a shed load of interrupts and the daughter board already has de-bounce capacitors, so no extra programming should be necessary. I've worked with the SIM 7600 before, and it's a nice bit of kit. My only worry in power supply, but there seems to be a ton of options on the Maduino, so hopefully one, or some, of them will work.. Notice that on the image above, all the text is reversed, which is because all the Goodness on this board is on the lower side as the daughter sits below the main board rather than on top. This way, we'll still have access to all the switches and blinkenlights on the Maduino.

Just for future reference, this is NOT going to be a smart phone, but neither is it a dumb phone as the onboard MCU could be programmed with all kinds of novel features. It could also be used with a PC based app by communicating directly with the SIM 7600 chip with AT commands. The world is our oyster.

So, here is the Maduino board:

..... and currently my daughter board is an exact overlay, which is great as a first stage into getting it into the shape of a brick. But, assuming the daughter board works ok, what next? I'm imagining refactoring the daughter board slightly to add castellations to interact with side panels and a base panel to enclose a suitable heavy battery and create the impression of brickness.