[SR-Users] Upstream ignores reply, because To-tag doesn't match?

Hi ppl, Scenario: invite from upstream is t_relayed to a client gateway. After `fr_inv_timeout` occurs, in a failure route I simply t_reply with "503 - Service unavailable", then exit(). However, the upstream provider simply ignores this reply, call doesn't hang up. This doesn't happen when the client's gateway generates identical 503 or other negative replies. I suspect this is happening because the To-tag in the Kam generated 503 reply doesn't match with the To-tag that previously was forwarded from the client's gateway to the upstream provider in the `180 Ringing`. Any suggestions what can be done about it? 2019/04/06 10:42:46.127177 65.39.xxx.xxx:5060 -> 208.72.xxx.xxx:5060 SIP/2.0 180 Ringing Via: SIP/2.0/UDP 208.72.xxx.xxx:5060;received=208.72.xxx.xxx;rport=5060;branch=z9hG4bK1982422573 Record-Route: <sip:65.39.xxx.xxx;lr=on;did=915.8af1> From: "SERGIU" <sip:514XXXXXXX@208.72.xxx.xxx>;isup-oli=00;tag=1975755942 To: <sip:514XXXXXXX@65.39.xxx.xxx>;tag=as2ab54180 Call-ID: DID-28270826(a)208.72.xxx.xx CSeq: 477023 INVITE Supported: replaces, timer, path Contact: <sip:1514XXXXXXX@65.39.xxx.xxx:5060> Content-Length: 0 2019/04/06 10:42:51.072516 65.39.xxx.xxx:5060 -> 208.72.xxx.xxx:5060 SIP/2.0 503 Service Unavailable Call-ID: DID-28270826(a)208.72.xxx.xx Via: SIP/2.0/UDP 208.72.xxx.xxx:5060;rport=5060;branch=z9hG4bK1982422573;received=208.72.xxx.xxx From: "SERGIU" <sip:514XXXXXXX@208.72.xxx.xxx>;isup-oli=00;tag=1975755942 To: <sip:514XXXXXXX@65.39.xxx.xxx>;tag=a6a1c5f60faecf035a1ae5b6e96e979a-8e82 CSeq: 477023 INVITE Server: KAM Content-Length: 0 Thanks, --Sergiu