Numerical data validation using VBA for combobox on form.

I have a combobox on a form that serves different purposes. Depending on the user selection, it might operator like a normal combobox where the user selects from a list- no problem. Other times the drop down is hidden and it acts like a text box and is designed to take in numbers- also no problem. When the combobox uses a list and the user tries to type something not contained in the list, an error is generated- Perfect!

How can I set the same thing up for when a number should be typed in? This should operate like the greater than/ greater than equal to etc data validation in normal excel.

Any ideas?

Hopefully I dont need to program something long and complicated- there must be a simple combobox property that I have overlooked!