i got a letter from dwp last week and it had the 0845 number to contact them.
Changing over thousands of phone lines is a big job and takes a while. Updating thousands of web pages, documents, forms, letterheads and the many other places these numbers are mentioned takes very much longer.
If you call one of the numbers that has been taken out of service you will now hear a message similar to:
"This number has changed. Please redial replacing 0845 with 0345. You have not been charged for this call."