You should hire counsel to review the thing you are contemplating signing, You should never sign anything you do not understand. Do not delay as the passage of time will negatively affect your rights.
IF you don't understand what was sent to you I would suggest you seek advice from an attorney. Just because a lawyer sends a letter to you does not necessarily mean you need to respond. It all depends on the circumstances and what is involved. You can certainly send a letter back asking for clarification. However, I would seek counsel first. It is imporatnt that you do not compromise your position.
Your question does not provide enough details about the letter, but if it is a bill from your lawyer, then it should list dates, activities undertaken, and applicable hours and rates and any additional costs (including use of paralegals or other parties with their charges separately detailed). It should be sequential and clear. If it isn't, then require that he resubmit it to you in a manner that is. If you have to pay within a certain period of time or interest penalties will be applied, then tell him/her that the time period you have to pay should not start to run until you receive the revised bill. If you talk to them on the phone, depending on the outcome of the call, you may want to send a follow up email or letter to the lawyer to confirm your understanding as to what he will do in response to your complaints.