It seems a feature is missing in V15 where the originator DID is not sent during a transfer. The transfer to phone only sees the middle party number Trunk ID and not the originator ID. Though this varies in carriers and with 3CX it doesn't work even after making all outbound parameter settings. The carrier is not blocking and was proven in testing on other systems. This could be a diversion header option that's needed in outbound parameters. Lots of articles about this.