I have been named the executor of my mother and by her will everything of hers was left to me. An investment?

firm that holds some of her mutual funds is insisting on my providing them an estate tax number in order to transfer the funds into a new account in my name. My mother has other mutual fund accounts with different investment firms and none of them required this estate tax number in order to honor my request to transfer the funds into my own account with them. The said investment firm that is insisting on it says that in Texas if the account is over 50,000 then it must pass through an estate number. However, once again the other funds that she had were above this dollar amount yet no estate number was required. Can you help clarify this? The Texas probate code is too bulky and I have tried to find this but have not located where it states this.