[Users] BYE before 200 OK?

[Michael Ulitskiy]

Hello,

Can someone please enlighten me on if this is allowed to send BYE
before 200 OK? I thought that BYE should be used only after dialog
established, but now I have a SIP PSTN termination provider and they
appear to be sending BYE with route headers right after 183 Session Progress
in some cases.
So the session looks like the following:
UAC                                OpenSER PSTN                       Provider UAS
INVITE to openser
                                         180 Trying back to UAC
                                         INVITE to UAS
                                                                                                180 Trying back to openser
                                                                                                183 Session progress back to openser
                                       183 Session Progress back to UAC
                                                                                                BYE to openser with Route headers
                                       BYE to UAC (loose_routing)
200 OK to openser
                                       200 OK to UAS
Is it ok?
And there's even more to it. I don't know if above is OK or not, but I would thought 
that now the session is ended. Nonetheless in exactly 50 seconds I'm getting
                                                                                                  183 Session progress back to openser
                                       183 Session progress back to UAC
-- in another 60 seconds
                                                                                                   183 Session progress back to openser
                                       183 Session progress back to UAC
-- this happens 6 times every 60 seconds and then
                                                                                                   408 Request Timeout back to openser
                                        ACK to UAS
                                        408 Request Timeout to UAC
ACK to openser

Do you think it's a provider bug? 
Another question is how openser is routing replies that cannot be matched to transaction
as I think transaction has already been deleted after BYE and latest 183 replies were routed
by some other principle. 
Could please someone comment on this?
Thanks,

See you later,
                    Michael