If you want to receive data and/or fax via GSM, you must get and use the incoming numbers provided by your GSM network. You will thus have separate incoming numbers for Voice (your current cellphone number), Data
9600 baud, Data 2400 baud, and Fax 9600 baud.
Someone sending you data or fax will thus have to dial one of these incoming numbers to do so. To send you a fax via GSM, they must thus use your incoming fax number etc.
The reason for this additional numbering that not all the analogue fixed line exchanges/switches can process and route the digital data that the digitally based GSM networks and cellphones use.
Somel GSM networks offers single numbering. Then you will have one number for all different kind of bearers like in a landline based ISDN application. The kind of the bearer is digital coded. Mostly it is not needed in industrial mashine to mashine communication. Single numbering can make trouble if you have designed your application local based on multi numbering and your client is supported by GSM operators that offers single numbering only.