--- res/res_pjsip_session.c	(Asterisk 13.13.1)
+++ res/res_pjsip_session.c	(working copy)
@@ -1998,2 +1998,11 @@
 		ast_exists_extension(NULL, session->endpoint->context, session->exten, 1, NULL)) {
+		size_t size = pj_strlen(&sip_ruri->host) + 1;
+		char *domain = ast_malloc(size);
+
+		if (domain) {
+			ast_copy_pj_str(domain, &sip_ruri->host, size);
+			pbx_builtin_setvar_helper(session->channel, "SIPDOMAIN", domain);
+			ast_free(domain);
+		}
+
 		return SIP_GET_DEST_EXTEN_FOUND;