This project uses the following sites and code as its base, either inspiration, snippets, or educational, during its evolution from super basic building blocks to the final code.
http://yesthomas.com/Other/201703-Arduino-A6-GSM.html
https://alselectro.wordpress.com/2016/09/14/gsm-a6-with-arduino-making-a-call-sending-sms/
http://makerangst.com/a6-gsm-gprs-module-send-an-sms/2
https://www.instructables.com/id/Mobile-Intercom-With-A6-Module-and-Arduino/