also, there must be a way of enable/disable for this, from the script.

5.4.1.7 Notification of Application Servers about registration status

During registration, the S-CSCF shall include a P-Access-Network-Info
header and a P-Visited-Network-ID header (as received in the REGISTER
request from the UE) in the 3rd-party REGISTER sent towards the ASs, if
the AS is part of the trust domain.

If the AS is not part of the trust domain, the S-CSCF shall not include
any P-Access-Network-Info header or P-Visited-Network-ID header. The
S-CSCF shall not include a P-Access-Network-Info header in any responses
to the REGISTER request.