Highlight numeric characters in a maskedtextbox

I am using a masked text box to allow users to enter a serial number. The problem is that this serial number has to be communicated over the phone and people seem to be having trouble differentiating between O and 0 and I and 1.

I thought that a good solution to this might开发者_JAVA技巧 be some way of highlighting which characters in the textbox were numeric characters (as opposed to alphabetic characters). Obviously there is no way to do this without using a richtextbox and then I lose the masking ability.

Can anyone think of a good solution to this? (Changing the font to one that has a slashed 0 would help but not for I and 1)

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Go for a font with a slashed zero and a 1 with serifs. To stop confusing the I with 1, you could use lower case letters to have a dotted i. Better even... avoid 1,0,I and O in your serial numbers...?