This has definitely been discussed before; unfortunately, I guess a search for "SMS" just gets swamped with complaints about the use of SMS-style abbreviations? So here's some to get you started:
http://www.8052.com/forum/read.phtml?id=122013
http://www.8052.com/forum/read.phtml?id=122254

In particular, see http://www.8052.com/forum/read.phtml?id=122184 which gives links to all the relevant specs.

Also, there is loads of stuff about this on the internet - just put "interface 8051 to GSM phone" into your favourite internet search engine...

I wouldn't be at all surprised if Siemens use the same AT Commands on their modules as on their phones - so try also searching for the modules: MC35, MC35i, TC35i, etc; you need the AT Command Set manual; eg,
http://uk.altavista.com/web/resul...gs=0&kls=1