Displayed number when Call Forward to external numbers

When making a call from external to an extention that is forwarded to a cellular I want the A-number to be displayed. This is used when someone calls my extention which is forwarded to my cellular. Than I want to se who is calling and not my DID number.

The problem is that my vendor has a whitelist on the SIP trunk only passing allowed numbers. The whitelist is based on the From header in the Invite. They told me to use P-asserted-identity and set the A-number there. This should display the A-number and the calls will be billed on B-number (from the From header).

On the outgoing trunk I have set the sendrpid=pai. This gives me the PAI in the Invite, but it is the same as in the From header.

Does anyone know how to do this:
From = B-number
P-asserted-identity = A-number

I’m running FreePBX 2.9.0.7 with Asterisk 1.8.5.0.

Please help!