can not use undo (vba excel)

Hello.

I made an vba excel application.
(when i do an entry in a specific cell, the cell is validated by an vba function and some network communication is made).

However, when i do an entry in such a cell and the vba functions are performed, i see that i can not use the "UNDO" button anymore.
(when i do an entry in other cells that don't call the functions, the undo button doesn't has that behavior)

very strange.
how does it come and what can i do?

thanks in advance

Oooh ... something in the back of my foggy brain says that you can 'bump' the undo list in your VBA function ... I'll try to figure that one out. It is a line of code you put in your function.

I must have been thinking of some other language ... check this link in this forum:
VBA Undo - Excel
It says that you have to implement special code that 'remembers' what was done by your macro so that it can be undone by your code. Sorry.