Well, my experiences are with Sagem MyX5-2. My purpose wasn't sending SMS, but downloading pictures - I bought it more for the digital camera than for the phone itself, but it appeared the "official" software doesn't work with "unofficial" cables (because it requires DTR-RTS shorted, which took some figuring out). Once I found out the stupid "protection" scheme, I went on errand to write software that does what the "commercial counterpart" does, except without "fake hardware flow control". (shortly after I released my program, the official one (upgrade) stopped caring about the "hardware control" and started working with "unofficial cables") Well, underway I found quite a bit about the way the phone talks with the world.
First off, it uses RS232, not 232C, so it did require a level converter. (for MCU, it seems you can just connect it...) Then, it uses seriously extended AT command set. It takes some figuring out which commands are just compatiblity stubs (i.e. always return error), and which ones are just gates into way deeper functionality. There is a group of commands that lets you control the keyboard remotely, so if everything else fails, you can fall back to "typing SMS" that way. The SMS/MMS command set is pretty general, but the actual format of the data you send/receive is quite obscure (I still didn't figure out the format of the data in which I get the images, I just look for a JPEG header and save everything after it), and it seems you separately "save the SMS in the phone" and "send saved SMS", and you have to identify the hash that corresponds to the new storage entry, probably by comparing against the old list.
This all is about a SAGEM phone and may or may not apply to Siemens... but may be of some help.