[Users] Caller-ID setup help

Hi, a Happy New Year to all. Looking for help to get our caller-id working for outging PSTN calls. We have 3rd party PSTN gateway service and all our callee gets the gateway phone number as CID for all of our sip calls to PSTN. I had implemented so far sip-rpid, remote-party-id, through avp, append_rpid_hf() and confirmed rpid headers on all pstn outgoing frames. However, callee only see the SP's PSTN gateways number as incoming caller-id. Is there other way of implementing caller-id for sip calls before I bring the subject to PSTN service providor? Or is it most likely SP's policy setup? Thanks in advnace. John K