Q:

.SMS file GSM 6 bit decoding

I have to decode a file .SMS witch is encoded in GSM 6 bit(the SMS is not set from a cell phone but from some sensors to a base station, an then I get the .sms files). Can you help me with this any idea or tip? Thanks! 
There is a lot about 7 bit and 8 bit but nothing about 6 bit. Is is possible that is an user-defined alphabet?

A:

3GPP TS 27.007 Section 5.5 gives the allowed coding schemes supported by the AT+CSCS command.
As far as I am aware, none of them are 6 bit.  But it does say that 

(conversion schemes not listed here can be defined by manufacturers)

So it seems that as you suspected, you may have a user-defined alphabet.
How to deal with this?  
If you can't get any information from the sender, then I would try fitting your messages to the first 6 bits of all of the officially supported conversion alphabets, to see if you can match all the characters.    
Consider the contents of the messages - are they a limited character set, e.g. all numbers (0-9)? 
It is possible that the sender may have used an official alphabet, maybe one that contains all the characters that they need in the first 6 bits, so they did not need to use the 7th and 8th bit.   

