ISP's that allow Indirect Access Numbers

I am currently with Virgin Media and use 18185 for my landline calls. If I move away from VM I don't want to lose my 18185 access but it's very difficult to find out which ISP's allow Indirect Access Numbers to be used. I believe that BT and Plusnet allow them, but what about others such as Shell Energy et al? Is there a website which has a definitive list of ISP/Indirect Access status?
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