I had this a few years back where I wanted to change two of the ISDN2e numbers (out of three and non-sequential), one to PSTN and the second to BTs Broadband voice (VOIP). It took a lot of calls and I believe I started a complaint (not at CEO level though) and then it was finally done manually. I will have a dig around to see if I can find any contact details but don't hold out much hope.
Recently, I then moved them both to a third party VOIP which had a few issues and I hate to think what might have happened if I had wanted to go from ISDN2e directly to a third party.
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