FROM vs. P-Asserted-Identity

[lowtech1]

hi everyone,
i got a question about the following setup:

CM: S8730-015-02.1.016.4/02.1.016.4-19308
SES: SES-5.2.1.0-016.4/SES-02.1.016.4-SP3b
outgoing routing:
internal H.323-->SES-->SIP trunk from SES to a SBC-->Nortel CS2k-->PSTN

we are using public/unknown numbering to setup outgoing CLI for the callcenter, e.g.:

NUMBERING - PUBLIC/UNKNOWN FORMAT
Total
Ext Ext Trk CPN CPN
Len Code Grp(s) Prefix Len
4 5234 21 xxxxxxxxxxx 11
(the x's are whatever we set up as CLI)

now i got informed by our legal dept. that it is a legal requirement that we send out user provided CLI only, not network provided CLI.
the CS2k guys are telling me that we come out as network provided on their side, not user provided and that i shall configure the PBX to send the real originating number in P-Asserted-Identity, while i can keep using pulic unknown in the FROM to have a corporate identity outgoing CLI.

Does anyone know how to do this? It feels like i went through the whole web for 3 times but could not find an answer on this.

any help is highly appreciated!