diff --git a/asterisk/IMS_ConnectionHandler.ttcn b/asterisk/IMS_ConnectionHandler.ttcn
index 58ca87a..4f87f95 100644
--- a/asterisk/IMS_ConnectionHandler.ttcn
+++ b/asterisk/IMS_ConnectionHandler.ttcn
@@ -452,13 +452,16 @@
 	}
 	for (var integer i := 0; i < lengthof(caddrs); i := i + 1) {
 		var ContactAddress caddr := caddrs[i];
-		/* "The UE must include the audio media feature tag, as defined in IETF RFC 3840,
-		 * in the Contact header field of the SIP INVITE request, and in the Contact header
-		 * field of the SIP response to the SIP INVITE request" */
 		if (not ispresent(caddr.contactParams)) {
 			Misc_Helpers.f_shutdown(__BFILE__, __LINE__, fail,
 				log2str(g_name & ": Received unexpected empty Contact attributes (omit)"));
 		}
+		/* GSMA FCM.01 3.2.3.3: "The INVITE request contains Within the Contact header [...] the IMS
+		 * Communication Service Identifier's (ICSI) for IMS Multimedia Telephony" */
+		f_sip_param_match_value_or_fail(caddr.contactParams, "+g.3gpp.icsi-ref", "\"urn\\%3Aurn-7\\%3A3gpp-service.ims.icsi.mmtel\"");
+		/* "The UE must include the audio media feature tag, as defined in IETF RFC 3840,
+		 * in the Contact header field of the SIP INVITE request, and in the Contact header
+		 * field of the SIP response to the SIP INVITE request" */
 		f_sip_param_match_value_or_fail(caddr.contactParams, "audio", omit);
 	}
 }