Because HMRC withdrew their Complaints Code of Practice (COP1) and replaced it with a fact sheet http://www.hmrc.gov.uk/factsheets/complaints-factsheet.pdf
I did some testing of the claims the factsheet made across the breadth of HMRC. Shall we say that the results were variable.

In the tax credits area my small sample displayed some variation in the story I was given by the Tax Credits Helpline as to whether they were geared up to receive complaints by phone or fax, so I asked for some definitive guidance.

I am told now that:

If you phone your complaint, the contact centre advisers will endeavour to resolve the complaint first, however, if this is not possible there is an escalation route between the Contact Centre and TCO that enables a TCO complaint handler to call a claimant back to discuss and resolve their complaint.

If you want to fax a complaint the number is 01772 235420.

But I expect you all knew that anyway..........