diff --git a/openbsc/src/gprs/gprs_gmm.c b/openbsc/src/gprs/gprs_gmm.c
index 82a4ca8..158f577 100644
--- a/openbsc/src/gprs/gprs_gmm.c
+++ b/openbsc/src/gprs/gprs_gmm.c
@@ -671,7 +671,16 @@
 #if 0
 			return gsm48_tx_gmm_att_rej(msg, GMM_CAUSE_IMSI_UNKNOWN);
 #else
-			/* As a temorary hack, we simply assume that the IMSI exists */
+			/* As a temorary hack, we simply assume that the IMSI exists,
+			 * as long as it is part of 'our' network */
+			char mccmnc[16];
+			snprintf(mccmnc, sizeof(mccmnc), "%03d%02d", ra_id.mcc, ra_id.mnc);
+			if (strncmp(mccmnc, mi_string, 5)) {
+				LOGP(DMM, LOGL_INFO, "Rejecting ATTACH REQUESET IMSI=%s\n",
+				     mi_string);
+				return gsm48_tx_gmm_att_rej_oldmsg(msg,
+								GMM_CAUSE_GPRS_NOTALLOWED);
+			}
 			ctx = sgsn_mm_ctx_alloc(0, &ra_id);
 			if (!ctx)
 				return gsm48_tx_gmm_att_rej_oldmsg(msg, GMM_CAUSE_NET_FAIL);