SDP not being modified by SBC in Teams deployment

Hi guys,

We are trying to implement Teams as per Oracle's Asia design (Proxy SBC & Downstream SBC) and we are facing a weird issue :

The calls fail and it seems that it's because the SDP is not modified in the SIP INVITE.

The call flow is the following : Teams -> Proxy SBC -> Downstream SBC -> PSTN

Here is the SDP of the INVITE ingressing Downstream SBC :

v=0
o=- 0 0 IN IP4 52.114.255.33
s=session
c=IN IP4 52.114.255.33
b=CT:1500
t=0 0
m=audio 51101 RTP/SAVP 104 102 9 111 18 0 8 103 97 13 118 119 101
a=rtcp:51103
a=ice-ufrag:nTe7
a=ice-pwd:o8iyhUo90kfdzd+Z/SbZOOOd
a=rtcp-mux
a=candidate:6 1 UDP 1258288639 52.114.255.33 51101 typ relay raddr 194.250.98.103 rport 41115
a=candidate:6 2 UDP 1258288126 52.114.255.33 51103 typ relay raddr 194.250.98.103 rport 49750
a=candidate:1 1 UDP 2130706431 10.82.54.199 50012 typ host
a=candidate:1 2 UDP 2130705918 10.82.54.199 50013 typ host
a=candidate:2 1 tcp-act 2121006078 10.82.54.199 50000 typ host
a=candidate:2 2 tcp-act 2121006078 10.82.54.199 50000 typ host
a=candidate:5 1 UDP 1694496767 194.250.98.103 41115 typ srflx raddr 10.82.54.199 rport 50012
a=candidate:5 2 UDP 1694496254 194.250.98.103 49750 typ srflx raddr 10.82.54.199 rport 50013
a=candidate:7 1 tcp-act 1684795902 194.250.98.155