Andy,
Yes the 'Redirected Call Forward' information can be passed to the carrier but as the original calling number is not in the range of DDI numbers you own the carrier will normally overwrite this with you main number or worse your billing number. The reason for this is to stop people accidentally or deliberately impersonating a different company.
Log onto your gateway and capture a "debug isdn q931" and have a look at the information the gateway is passing to the carrier, you may need to change the mask on the pattern or route groups. It's easy to test this by putting in a specific pattern for your mobile that points down a test route list / group to the gateway.
With my mobile I get the correct original called number and a little arrow which lets me know it has been forwarded from my desk phone.
If you post a trace we can try and help you.
Hope this helps,
Craig
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