--- res/res_pjsip_sdp_rtp.c	(Asterisk 13.6.0)
+++ res/res_pjsip_sdp_rtp.c	(working copy)
@@ -390,2 +390,5 @@
-	rtpmap.param.slen = 0;
-	rtpmap.param.ptr = NULL;
+	if (!pj_stricmp2(&rtpmap.enc_name, "opus")) {
+		pj_cstr(&rtpmap.param, "2");
+	} else {
+		pj_cstr(&rtpmap.param, NULL);
+	}