It could be that you are a witness to some event that they represent someone else for, they could represent a creditor of yours, or many, many, many other reasons.
If the lawyer charges an hourly fee, the lawyer will bill you for small tasks like writing emails to you and answering your telephone calls. Some lawyers charge for their time in six-minute increments, and will round up. For example, if your lawyer charges $250 per hour, a ten-minute phone call may cost you $50.
So often when a lawyer does not return your call for a few days it may simply mean your lawyer is busy getting some important work done in your case or in another client's case. There is nothing going on with your case.
Once a month is a good rule of thumb if things are slow, but if you are preparing for trial or in my case an administrative benefits hearing, the contact with you and your attorney should be more frequent and specifically scheduled.
For example, in a company or corporation, a general retainer fee would include charges for contract drafting, board resolutions, certifications, and the likes. Retainer fees vary greatly depending on the client's situation and most lawyers require a retainer agreement or contract.
As a general rule, give your lawyer 1-2 days to return your non-urgent phone call; very often, your lawyer may be out of the office for a half-day, whole day, or longer if he/she is in court.
Throughout the process of getting your financial settlement after becoming injured, there may be periods of time that you do not hear from your attorney. Although this can be unnerving, it is a normal part of the legal process.
If it is an emergency, then a weekend call may be appropriate. If there is something life-threatening about the situation, you should call. If it is something that would have a negative consequence that couldn't later be fixed, then you would want to give your attorney a call.