Show orginal CallerID when forwarding to Mobile Phone

Hi

Could anyone point me into the right direction what to change to have the original caller ID forwarded to the mobile phone the extension has set?

More details:

$OG-CallerID calls extension 200, extension 200 has a forward set to his Mobile Phone (not on the PBX), instead of seeing the $OG-CallerID on his phone, he can only see his own Outbound Caller ID number calling him.

Our provider allows sending other numbers.

Thanks!