I have been experimenting with the SIM20 this week, very impressed with it once I had worked out the datasheet,
some parts of which seem to say the module is more intelligent than the user. i.e.
3.3 Turn-on and turn-off of the SIM20
3.3.1 Turn on SIM20
SIM20 can be turned on automatically when power is supplied.
3.3.2 Turn off SIM20
Users can turn off SIM20 module by cutting off power supply.