I had to call HMPO on three consecutive days in February.
+443003300901
Frustratingly, after the first two calls and assuming each time that the matter was resolved, I received an email requiring further action.
Finally resolved the issues and clarified exactly what was required then sent my old passport directly to them with priority EMS from Thailand to the UK.
My advice is not to try and understand the logic nor question why the request was not made earlier - simply clarify what is needed then do it - without getting frustrated with the person on the other end of the call..