I am trying to add P-Access-Network-Info header to REGISTER request using CSIPMessageElements::SetUserHeadersL, but it leaves with -6. If I do that with a fake header name "P-Header", it works ok. The value for that header is "3GPP-GERAN;cgi-3gpp=012345cb21932" which seems to me syntactically ok. Also, can't see reason, why would SIP stack check syntax of user header.
Is P-Access-Network-Info disallowed from user headers? It is not listed in SDK help to CSIPMessageElements as disallowed.
What other way can it be added to REGISTER request?
Thanks for any help in advance.