--- channels/chan_sip.c	(Asterisk 13.13.1)
+++ channels/chan_sip.c	(working copy)
@@ -7394,6 +7394,12 @@ static void try_suggested_sip_codec(stru
 
 		ao2_ref(fmt, -1);
 	}
+
+	/* The original joint formats may have contained negotiated parameters (fmtp)
+	 * like the Opus Codec or iLBC 20. The cached formats contain the default
+	 * parameters, which could be different than the negotiated (joint) result. */
+	ast_format_cap_replace_from_cap(p->jointcaps, original_jointcaps, AST_MEDIA_TYPE_UNKNOWN);
+
 	ao2_ref(original_jointcaps, -1);
 	return;
  }