From gprs_gmm.c

                        char mccmnc[16];
                        snprintf(mccmnc, sizeof(mccmnc), "%03d%02d", ra_id.mcc, ra_id.mnc);
                        if (strncmp(mccmnc, mi_string, 5) &&
                            (sgsn->cfg.acl_enabled &&
                             !sgsn_acl_lookup(mi_string))) {
                                LOGP(DMM, LOGL_INFO, "Rejecting ATTACH REQUESET IMSI=%s\n",
                                     mi_string);
                                return gsm48_tx_gmm_att_rej_oldmsg(msg,
                                                                GMM_CAUSE_GPRS_NOTALLOWED);
                        }

It needs to be commented out to allow roaming IMSIs to receive PDP contexts