CMS have an online self service website that you may wish to join. Can be useful for uploading documents.
Guessing that you are the RP (Resident Parent) and seeking Child Maintenance?
I made an application I end of November last year. Took till mid January this year to receive an assessment. Not too bad considering that Xmas and New Year were in the middle.
Whenever I called I used to speak to different person as assigned case worker not always available. Some seemed more knowledgeable than others, but it was resolved in reasonable time overall.
Once you have received CMS assessment it will include details of what to do if you are unhappy with the service or you disagree with the assessed figures.