REGISTER Request with Cseq=1 should ALWAYS be proxied by SBC?

Hi Fellows,

I hope you are ok.

I am having an issue with one of my devices registering with the SBC and Registrar Server.

What am seeing is that when UAC sends a new REGISTER request with Cseq:1 we are not proxing this request and then the UAC sends a wrong Authentication URI, causing my Registrar Server to reject with SIP 403.

I am trying to see if the behavior of the UAC is because it is expecting to receive the WWW-Authenticate challenge, cause it sent a REGISTER with Cseq=1.

I was trying reading the 10.2 Constructing the REGISTER Request and 10.3 Processing REGISTER Requests and the rest of the RFC, but is not clear if the SBC should have to proxied the REGISTER always that Cseq=1.