The way the breakout for the Sim800L is wired, the module turns on as soon as it gets power. The plan was to tell the module to power off when not being used then use the Reset pin to bring it back to life. I wrote some quick code, wired up the Reset pin to a digital IO pin on the Teensy and I could turn it off, then reset it to bring it back to life so moved on to the rest of the code thinking this was sorted - more on this later.