Please remember that if you didn't have a GSM attach, you are not able to attach to GPRS unless you are using packet only mobile. So once you didn't attach to SGSN, that means you are unknow by SGSN, even SMS over GPRS is enabled in your operator and even HLR still have SGSN address, you are in "IDLE" state in SGSN, you are not able to get ANYTHING from SGSN before you re-attach to SGSN.