Grantor vs Non-Grantor Trusts

I received a call a couple of days ago from previous clients who asked whether or not they needed to get a tax number for their trust.  They did not understand that as their trust was a revocable trust (also known as a grantor trust) meaning they had full control of trust assets, their social security numbers were automatically the tax identification numbers for the trust and any trust income should be filed on the grantors' 1040. Only when a trust is or becomes irrevocable (or is known as a non-grantor trust) do the trustees acquire a separate tax identification number and file a 1041 as long as the irrevocable trust is in existence.