Internal energy change during phase change
You are considering the $\Delta H$ and the $\Delta U$ between the following two thermodynamic equilibrium states:

State 1:  1 mole of ice at 0 C and 1 atm.

State 2:  1 mole of liquid water at 0 C and 1 atm.

The relationship between $\Delta H$ and $\Delta U$ at constant pressure is:
$$\Delta H=\Delta U + p\Delta V$$where V is molar volume.  What is the molar volume of ice?  What is the molar volume of liquid water?  What is $\Delta V$?