Where to set AT commands for modem? I'm trying to use a modem with an Apple MacBook Pro running Sierra (10.12.6) and I've been given the following instructions: 

The user will have to create a standard 19200bps modem on 
the COM port used by the Iridium device with extra 
initialization command as: AT+CBST=71,0,1


But don't know where to configure this. I have a phone attached via USB that I can use to dial and connect to an ISP, but can't find anywhere to configure the AT command set.
You have two options to &quot;talk&quot; to your modem on serial ports on macOS:

Minicom
Screen

Screen is built in, and minicom is 3rd party.  Before using either of the tools, you will need to know the device your modem is attached to.  It will be listed in /dev/ttySX where X is the number of the actual device (most likely /dev/ttyS0).  Verify for your machine.
Using Minicom
Minicom is a small, free communications program that's been around for decades and is quite solid.  I personally prefer it over screen.

Homebrew:  brew install minicom
MacPorts:  port install minicom

This assumes you've already installed the Homebrew or MacPorts software.
It's a simple ANSI graphic interface that will allow you to set your serial device, flow control, port speed etc.

Once you have it set, you will be connected to the device on your serial port you can then send your AT commands to your modem by typing AT+CBST=71,0,1

Using Screen
You can use the built in screen to issue the commands (though admittedly, I am not very familiar with the utility as I favor Minicom)
You can connect to your device by issuing the command (assuming your modem/serial port is configured to 115200bps, 8 data bits, no parity bit and 1 stop bit or more commonly known &quot;115200 8N1&quot;):
screen /dev/ttySX 115200,cs8,-parenb,-cstopb,-hupcl

Where /dev/ttySX is the serial device connected to your modem as before.  This example sets the comport to 15200 8N1.
And just like in the Minicom example, once connected, just send your AT commands to the modem.