When the From and To messages of the Bye message are received in reverse.

Hello

A-----> B(SBC) --------------> C

                                             INVITE

                                      <--------------------

                                              BYE

The first INVITE sent to C by SBC is shown below.(example)

-->

INVITE sip:2345678@2.2.2.2:5060;user=phone;sat-hops=1 SIP/2.0

From: <sip:1234567@1.1.1.1:5060;user=phone>;tag=c4-45026-e9cf2e-7d04b4b9-e9cf2e

To: <sip:2345678@2.2.2.2:5060;user=phone>

......

The call completed successfully, and C sent a BYE message to B.

However, the BYE message From / To is reversed from the INVITE From / To.

The BYE message is:

BYE sip:1.1.1.1:5060;transport=udp SIP/2.0
To: <sip:1234567@1.1.1.1:5060;user=phone>;tag=c4-45026-e9cf2e-7d04b4b9-e9cf2e
From: <sip:2345678@2.2.2.2:5060;user=phone>;tag=sbcsipuas_1_C26194_20191114215150768_hkb02sb01