Old thread but I thought I'll post a reply for anyone else searching into this. I needed to have a remote sms alarm from my boiler, and I had quite a bunch of old siemens MC35i modems around. It turned out sending sms messages from the modems internal storage was quite straightforward, basically just a oneliner serout command.
Code:
serout 1,N1200_4,("AT+CMSS=1,+35840XXXXXXX,145",CR) 'AT command to send SMS from storage slot 1, phone number in international format, and 145 as address / phone number format "magic number"'
Inserting messages into the storage or writing variable messages is a bit more complicated at least with this siemens modem, as they rely in interactive serial console (thought one could assume ideal conditions and just pause the program instead of parsing the replies from the modem..), so since only needing a general alarm message I could use a pre-recorded sms.. which I inserted just using serial terminal.
On this modem the serial port's DSR and CTS lines had just enough capacity to supply the picaxe board with just the axe mostly napping and shortly flashing an indicator led, so just the modem's power supply was required.
At least around here the 2G network is likely not going away anytime soon. They are phasing out 3G, but 2G has so many devices relying on it that I doubt it'll be going away anytime soon.