SBC sends 503 Service Unavailable instead of 180 Ringing

Hello,

in 80 % of this call scenario SBC doesn't pass 180 Ringing to the other side, it sends 408( there is sip-manip changing 503 to 408) instead. I t looks like some resouerces are missing but no idea which, since there are a lot of clients on this SBC(provider) and only thiss one is complaining. I have an example of KO call and of OK call. I changed public IP addresses and names

This is OK call; 2.2.2.2 and 3.3.3.3 are sip-interface on our SBC. Messages are:

2) INVITE return to call flow diagram return to call flow diagram

Time: 16:24:20.503 (less than 1 ms)

Verifier: MXFUBR2

Header: INVITE sip:289991497@2.2.2.2:5437 SIP/2.0