Unfortunately, I am not familiar enough with BASCOM or any other variant of Embedded BASIC to be of much help in composing the required code.
However, I would suggest attaching your GSM/GPRS module to your PC's serial port and using a terminal emulation software like Putty to test and troubleshoot the required AT Command sequences.
Once you have determined the required AT Command sequences and their order of issuance, incorporating them into your BASIC code should not be a major issue.
If your GSM/GPRS module only provides TTL serial output, you will need to utilize a RS-232 Transceiver, like the MAX232, before interfacing the GSM/GPRS module to your PC's serial port.
You'll need to choose the appropriate RS-232 Transceiver based on the allowable voltage levels of the GSM/GPRS modules TTL interface, if an RS-232 interface is not provided.
BigDog