I cannot dial a 1300 number from a Telstra TMS Sim. I get the phone message "The phone number you dialled is not in service"

Telstra TMS Sim supports 1300 dialling.  

If you get a "Phone not in service" message when calling a known working 1300 number, check if a + is automatically added when you dial on the Iridium handset.  

To remove the +, navigate through the phone menu Setup> Number Entry and select Do Not Prefix.  

Each time you dial now, phone number will not be prefixed with a +.  

This should allow you to successfully call a 1300 number.