Phoning HMRC solicitors at Bush House

I am trying to phone HMRC solicitors at Bush House London WC2B to follow up on a JR pre action protocol letter I sent to which I have had no response. 
The best number I have managed to track down is the one on the Law Society website: 03000 586705. However when I call I get a recorded message stating: ‘We’re sorry your call did not go through. Please try your call again later’. The message makes not mention HMRC solicitors.
Can anyone confirm if this number is correct or if there is a better number?
Thank you. Gail