First of all, an excellent article on power saving by Nick Gammon:
https://www.gammon.com.au/forum/?id=11497
If you use a 3231 RTC you can put everything to sleep, turn the SIM900 off, etc. The RTC can wake the microcontroller on an interrupt, in turn that can turn everything on. Once all is up and running send the information, reset the alarm, turn everything off and go back to sleep.