--- channels/chan_sip.c	(Asterisk 13.6.0)
+++ channels/chan_sip.c	(working copy)
@@ -12799,3 +12799,3 @@
 		ast_str_append(a_buf, 0, "a=rtpmap:%d %s/%u/2\r\n", rtp_code, mime, rate);
-	} else {
+	} else if ((35 <= rtp_code) || !(sip_cfg.compactheaders)) {
 		ast_str_append(a_buf, 0, "a=rtpmap:%d %s/%u\r\n", rtp_code, mime, rate);