Frogger
I assume that you are currently receiving either IB, IS due to Incapacity, or SDA and the letter is informing you that you will shortly be transferred to ESA.
The phone call is to see whether you have any question regarding the transfer, if you do then you should contact them, if not, then you don't need to and they will proceed to the next stage, issuing an ESA50.
You might want to have a look at our ESA Claim guides, see
www.benefitsandwork.co.uk/help-for-claimants/esa Gordon