Our lawyer withheld money to the Medical group that handled my Dad's bills and now this group is asking him to pay the balance.

In a typical personal injury case, the attorney pursuant to the fee agreement(retainer) would receive 1/3 of the settlement, then the doctor(s) and other unpaid medical bills where there is a lien are paid and then the remainder of the settlement would be paid to the client.  If the settlement isn't sufficient to pay all the medical providers who have liens, sometimes through negotiation, they will agree to a lesser amount.  The problem here is to determine why the attorneys have not paid the balance provided that the balance is subject to the lien.  If the attorneys continue to refuse to communicate with you, tell them in writing that you will contact the State Bar.  Most likely, they will respond once you tell them that.  If the balance is not subject to the lien, there is probably some general language in the lien that the patient/client is responsible for the medical bills.  However, at this point, your focus should be in determining why the attorneys have not paid the balance.