Dereham Royal Mail Delivery Office | Sorting Office

I just wanted to tell the parcel delivery service that I would not be there for the delivery of a parcel which may not go through letter box and would they please keep it until I returned home.I spent some time listening to recorded messages without learning anything,for which I have paid.I then spent more time searching on Google for phone number of Dereham sorting office but had no luck as call was not answered and then cut off. I have been unable to speak to anyone even after all that effort.
How long are parcels kept in Dereham sorting office??

This complaint is about the ability to contact the sorting office,not about the post office.