I imagine the simplest thing would be to have the forwarded calls appear to be coming from the phone # of the iPhone, but I am unsure how to do this.
Unfortunately, this cannot be done.
Your iPhone is not what is forwarding your calls; it’s the carrier. The way phone calls work is that the carrier takes the call data that has destination embedded in it data packets and forwards that data to the the device identified by the SIM on their network. When you forward, you tell the carrier to change that destination to somewhere else.
If you want to change the number that appears, you would have to “take ownership of the call,” modify the caller ID payload then transmit the new data packets to a new destination. This isn’t possible with an iPhone