surrogate-agent registration cache lookup keying off the P-Preferred-Identity header

Hello,

I created a surrogate agent for a PBX as is described in the BCP SIP Trunking Configuration for Service Providers. Now my problem is that INVITES from the PBX fail when the FROM is not the  same as the surrogate registration user. When the FROM is the same it works fine. The INVITE contains a P-Preferred-Identity header (with the registration user) and I presumed that it would work also, because the BCP says "For sessions initiated from the PBX, the Net-Net SD will perform a registration cache lookup keying off the P-Preferred-Identity header (if present) or From header."

Does someone knows if I need to add additional configuration so the SD will use the P-Preferred-Identity header to do the registration cache lookup?